The Description of Akshauhinis is identified as 21870 Chariots, 21870 Elephants, 65610 Horses and 109350 foot soldiers.
The Pandavas and Kauravas had more troops available than traditional number. The Mahabharata slokas have three meanings (according to Madhavacharya in Mahabharata Tata Parnirnaya the oldest extant commentary on Mahbharata more than Three). We will look at three: Astikadi (As-is), Uparichariradi (Transcedental) and Dharmic (Not exactly religious, more way of life/honor definition.)
The War is a Dharma-Yuddha. The name of Book is Jaya, the Great Bharata. The Jaya in question is the name of Yudhishthara (his personal name used by the One of Three Krishnas of the Epic, Krishnaa viz reference in Virata Parva, it is very early on).
Jaya or Great Bharata is Yudhishthara the one who has 40K verses of conversation (half the book, Krishna is present in book for less number of verses that Yudhishthara conversations).
So Dharmaraja Dharmaputra Dharma on earth is the TRUNK of Mahabharata (AdiParva first chapter), and Dharma's Katyapadi number is 18.
18 is all throughout book.
Pandu goes to Forest for 18 years. Kunti dies 18 years after War; Krishna 18 years after that. Duryodhana is 18th family destroying King. There are 18 Akshauhinis! There is 18 everywhere in the book.
That 18 is not Astika-di but Dharmic. It is pulling attention that we are not fighting an ordinary war but Extraordinary Dharmic War.
So, Pandavas are assigned 7 (again an important number) and Kauravas are assigned 11 (again an Important number, look at meaning of these numbers in Vana Parva Ashtavakra Nandi discussion chapter).
But Pandavas does not have mere 7: Their banners include has One from Kashi, One from Shivi lead by Yudhishthara's eldest son Satyaki (hence his vote is asked for War or Peace), third one by Virata Matsaya, Fourth one is Chedi Dhrstaketu, Fifth is the Kekaya brothers, Sixth is the Magadha Jarasandh's son Sahadeva, Seventh is Chekitan (which might be same as Kekaya Akshuahini) plus 140K warriors from Pandya Malaydhvaja Father in law to Arjuna plus seventh one from Karusha lands/Dasarna lands and extra Rakshasa Akshauhini of Ghatotkacha killed by Asvhathhama on 14th day.
Also Pandava numbers have fixed length and followed the 1:1:3:5 formation. and eventually Akshauhini size of 218,700 soldiers.
Kauravas had 10K Kuru princes (sons of 100 rather 192 counted Kauravas) and 55 to 60 lakh soldiers under them. These are then divided into Kaurava akshauhini sizes, their akshauhinis also called Varuthinis are much larger almost 2 times plus than Pandava Akshauhini and thus around 5 lakh soldiers in each Varuthini.
That makes 11 and 7 which makes 18 is a play by poet.
On 14th day, Arjuna killed 8 Kauarava akshauhinis, but the report is by Krishna who is using Pandava formation numbers; aka Arjuna, Bhima, Dhrishtadamyuna, Satyaki on that day killed 8 Akshauhinis or some 17 lakh soldiers. (in Kaurava parlance they lost some 3 akshauhinis and change)
Other major Kaurava losses, Bhima killed 3 Akshauhinis on 17th morning along with Dushasana.
On last day, Salya still brought in 3 Akshauhinis to battle, 3 by Pandav estimation, Salya has 2 lakh horses left, full 3 Akshauhinis, but less number of elephants and Rathas. but counted as Three on Pandava count...
In actual terms, Duryodhana had around 22-23 akshauhini force. He also mentions in Udyoga Parva that his army is 3 times Pandava size.