People are vastly overestimating sizes here. 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 Rohirrim ambushed them.
At the Battle of the Hornburg, 10,000 is the number most thrown around. 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 forest 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 bringing Isengard's full army to 20,000.
As for Mordor. The main force that attacked Minas Tirith in Pelennor Fields, contrary to popular belief, didn't come from Mordor, rather Minas Morgul, and was led by the Witch-king. Doing pixel measurements and given estimates the main orc army at Minas Tirith was around 30-40,000, 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 Oliphaunts to Pelennor Fields, has around 10-20 and say each one carried 2 dozen men. That's 20 times 24 which is 480 men on the Oliphaunts. As for their land army I've heard that around 3000 men were at the Black Gate so I’ll be generous and say another 6000 fought on Pelennor Fields. So including the forces in reserve in Mordor I’d say that the Haradrim had around 10,000 men and 10-20 Oliphaunts.
The Easterlings were 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 were 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 Battle of Pelennor Fields as we know that Rhûn has huge manpower and the only reason there would only be 5,000 in Mordor was if the main army was killed at Pelennor Fields. 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 War of the Last Alliance 600,000 orcs were used by Sauron. However this was before significant losses like losing the orcs of the Misty Mountains and Dol Goldur, so I'd say at the time of The 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 Oliphaunts
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, etc.