People are vastly overestimating sizes here
Ok so I’ll go in order of battles and then make a conclusion
At the battle of Amon hen we see a large scouting party of no more than 100 uruk hai about 20-30 escaped with merry and pippin after joining up with the orcs they likely swelled to 50 orcs and Uruk Hai they where killed when the rohirim Ambushed them.
At the battle of the Hornburg 10,000 is the number most thrown around and I agree it’s heavily implied in the movie that this was Isengard at full capacity due to them having to expand industry into fangorn Forrest and as stated in the movie, having trouble arming the Uruks. I would estimate that no more than a few dozen Ents attacked Isengard and the garrison (including orcs in the pits) was around 10,000 brining Isengards full army to 20,000.
As for Mordor. The main force that attacked Minas Tirith in the pelennor contrary to popular believe DIDN'T come from Mordor rather Minas Morgal and was led by they the witch king doing pixel measurements and estimates the main orc army at Minas Tirith was around 30-40,000 and I’d say the main force that marched out of minas Morgul was around 45-50,000 due to expending men at osgiliath. So minas Morgul 50,000 orcs.
The Haradrim assuming they brought their full force Of oliphants to the pelennor has around 10-20 and say each one carried 2 dozen men that’s 20 times 24 which is 480 men on the oliphants. As for there land army I’ve heard that around 3000 men where at the black gate so I’ll be generous and say another 6000 fought on the pelennor so including the forces in reserve in Mordor I’d say that the Haradrim had around 10,000 men and 10-20 oliphants
The Easterlings where harder as most of their forces participated in the war in the north however a significant force did take part in the southern theatre
It was mentioned that 5,000 Easterlings where in Mordor at the battle of the black gate as this was only a reserve I’d say AT LEAST 20,000 took part in the pelennor as we know that Rûn has huge manpower and the only reason there would only be 5,000 in Mordor was if the main army was killed at the pelennor so I would put the Easterlings at 25-30,000 at least.
Next is the corsairs it was mentioned that they would plunder southern Gondor with 50 ships assuming that these ships where all the same size as the ones in the movie and calculating that each ship could hold 200 at full capacity 200x50 is 10,000 so that’s where I would max out the corsairs strength
Finally Mordor in the movie it’s said that there are 10,000 orcs in the plains of Mordor I’d say that’s way too low considering minas Morgul is a small city and had 45-50,000 whereas Mordor is an entire region I’d say Mordor could field at least 100,000 orcs but during the was of the last alliance 600,000 orcs where used by Sauron however this was before significant losses like loosing the orcs of the misty mountains and dol goldur so I’d say at the time of return of the king Mordor could field 300,000 orcs, trolls, Uruk Hai at full capacity
So that bring our totals at full capacity
Minas Morgul: 50,000
The Haradrim: 10,000 with 10-30 oliphants
The Easterlings: 30,000 (at least)
The Corsairs: 10,000 men and 50 ships
So to answer your Question
Isengard could call upon 20,000 fighting orcs AT FULL CAPACITY
While Mordor and her allies could call upon around 500,000 men, orcs, ect