The Counter, The Reckoner, The One whose count of things is known to him.
The One who considers all things in the universe and knows the details of every moment or change. He comprehends everything and possesses all knowledge. Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ knows every deed, and written details are recorded by His angels.
From the root ha-sad-ya (ح ص ي), which has the following classical Arabic connotations: to number, count, reckon, compute to collect in an aggregate by numbering to register or record something to take an account of something to comprehend or know altogether to possess full comprehension.
Among the disputed names:
As we've mentioned in previous names, different scholars have different criteria for what qualifies as a name of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ. Of the 99 names, 81 are explicitly mentioned in the Qur'an. Therefore, you won't find Al-Muhsi recognized as an official name by certain scholars. This list includes Ibn Uthaymeen, Ibn Hazm, and Ibn Hajar, among others. However, others, such as Ibn Arabi, Imam al-Bayhaqi, and al-Ghazali, have included this name in their lists.
Abu Huraira reported Allah's Messenger (ﷺ) as saying: There are ninety-nine names of Allah; he who commits them to memory would get into Paradise. Verily, Allah is Odd (He is one, and it is an odd number), and He loves odd numbers. And in the narration of Ibn Umar (the words are): "He who enumerated them."  In the hadith, it uses the word with the same root (أَحْصَا - ihsa), meaning computation of some kind. To enumerate or to count.
What is meant by referring to Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ as the all-enumerating, the numberer, or the counter? It is an attribute of His knowledge, much like Al-Alim (The All-Knowing), Ash-Shaheed (The Witness), or Al-Khabir (The All-Acquainted). The difference is Al-Muhsi is often described more analytically. He has knowledge of all things. He takes into account each separate item and stores it in His ledger. Every breath inhaled and exhaled, every leaf that falls, and the movement of every atom is accounted for. Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ knowledge is infinite, He knows a thing through and through.
Some commentators mention He knows all the things that are possible, for everything that is possible. So, He not only knows everything but knows what it could have been. He's calculated the infinite possibility of every choice, and the downstream effects caused. There is nothing unknown to Him سُبْحَٰنَهُۥ وَتَعَٰلَىٰ.
Liya'lama an qad ablaghoo risaalaati rabbihim wa ahaata bi maa ladaihim wa ahsaa kulla shai'in 'adada
"That he may know that they have conveyed the messages of their Lord; and He has encompassed whatever is with them and has enumerated all things in number." — (Qur'an 72:28)
Wa kulla shai-in ahsai naahu kitaa ba
"But all things We have enumerated in writing." — (Qur'an 78:29)
Counter of deeds:
Wa wudi'al kitaabu fataral mujrimeena mushfiqeena mimmaa feehi wa yaqooloona yaa wailatanaa maa lihaazal kitaabi laa yughaadiru saghee ratanw wa laa kabeeratan illaaa ahsaahaa; wa wajadoo maa 'amiloo haadiraa; wa laa yazlimu Rabbuka ahadaa (Section 6)
"And the record [of deeds] will be placed [open], and you will see the criminals fearful of that within it, and they will say, "Oh, woe to us! What is this book that leaves nothing small or great except that it has enumerated it?" And they will find what they did present [before them]. And your Lord does injustice to no one." — (Qur'an 18:49)
The knower and bestower blessings:
Wa aataakum min kulli maa sa altumooh; wa in ta'uddoo ni'matal laahi laa tuhsoohaa; innal insaana lazaloo mun kaffaar (section 5)
"And He gave you from all you asked of Him. And if you should count the favor of Allah, you could not enumerate them. Indeed, mankind is [generally] most unjust and ungrateful." — (Qur'an 14:34)
From this ayah, we realize that we probably don't understand the extent of Allah's blessings. We're unable to count them all. If every reward or blessing required us to praise and exalt Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ we would fall short. We are forever indebted to His continuous favors, kindness, and mercy.
The knower of all people:
Innaa Nahnu nuhyil mawtaa wa naktubu maa qaddamoo wa aasaarahum; wa kulla shai'in ahsainaahu feee Imaamim Mubeen
"Indeed, it is We who bring the dead to life and record what they have put forth and what they left behind, and all things We have enumerated in a clear register." — (Qur'an 36:12)
Lillaahi maa fissamaawaati wa maa fil ard; wa in tubdoo maa feee anfusikum aw tukhfoohu yuhaasibkum bihil laa; fayaghfiru li mai yashaaa'u wa yu'azzibu mai yashaaa u; wallaahu 'alaa kulli shai in qadeer
"To Allah belongs whatever is in the heavens and whatever is in the earth. Whether you show what is within yourselves or conceal it, Allah will bring you to account for it. Then He will forgive whom He wills and punish whom He wills, and Allah is over all things competent." — (Qur'an 2:284)
This name reminds us that Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ knows you and me. If you feel unheard, small, or not counted for, remember Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is and always has been watchful over you. We all matter to Him, and He has accounted for us and guided us to live fruitful lives. He has not forgotten anyone. He knows everyone that has walked this earth - past and present.
In explaining each name, we try to find a way to connect the knowledge of the name to our everyday life. This often includes providing a reflection to consider how we can live and exemplify the same beautiful attributes to try and better ourselves. However, for the name Al-Muhsi, our access is limited. Awareness of this name is enough to have a meaningful impact in your life. Think of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ when confronted with a decision and ask what you believe would be most pleasing to Him.
As a comparison to try and illustrate the scale at which Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ intimately knows every person, we examine our limited social networking abilities. It was proposed by an anthropologist named Robert Dunbar that the number of meaningful relationships we could make was capped by the size of our brain's neocortex size - that there's a natural limit to the number of friendships we could actively manage. His proposed number is coined as "Dunbar's number" and suggests we could hold about ~150 relationships (a meaningful relationship is informally defined as the number of people you would not feel awkward about joining for a coffee or some get-together). Now, every relationship is not made equal. Of these 150, only 5-10 may be your core support clique. You frequently interact, share bad news, and know what's happening in each other's lives. The second group may be 10 people you consider your close friends. The further out you go, the weaker the bond. Outside this 150+ boundary would be acquaintances - people you know but aren't in close contact with.
Now, this number is disputed by some researchers - social sciences are not hard science. But the proposed number remains small (i.e., less than 400-500). Now, imagine the vastness and capability of Allah's knowledge when He tells us, "And with Him are the keys of the unseen; none knows them except Him. And He knows what is on the land and in the sea. Not a leaf falls but that He knows it. And no grain is there within the darknesses of the earth and no moist or dry [thing] but that it is [written] in a clear record." (Qur'an 6:59)
He not only knows each of us by name but remembers our prayers, our deeds, and what we conceal in our hearts. This is the vastness and complete knowledge of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ and what is conveyed through the meaning of His name Al-Muhsi.
 Sahih Muslim 2677a