Let's begin with Gondor ! I base my estimation on quotes from The Return of the King :
GONDOR
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Lossarnach : 2000
"Forlong the Fat, the Lord of Lossarnach, with two hundred well-armed men bearing battle-axes" Book V, Chap 1 it's mentionned that these 200 soldiers only represented one tenth of Forlong army.
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Ringlo Vale : 300
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Morthond, "
Blackroot Vale" : 500 archers
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Anfalas :
"long line of men of many sorts, hunters and herdsmen and men of little villages, scantily equipped" I estimate that represent like 100 or 150 men
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Lamedon : "
a few grim hillmen without a captain" someting like 50 guys
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Ethir Anduin : "Fisher-folk of the Ethir, some hundreds, or more" +- 100
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Dol Amroth : 700 men-at-arms and
" a company of knights in full armour" maybe 400
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Minas Tirith : Regarding the size of the city, the decline period for Gondor, i think there was approxiamtely 2000 soldiers to guard the fortress
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Ithilien :
"indeed all the survivors of the foray were now gathered on the slope nearby, two or three hundred strong" Book IV, Chap 5
At the Pelennor Field batlle, Aragorn brought 1000 soldiers from the Southern Fiefs.
"Four thousands I sent marching from Pelargir through Lossarnach two days ago" and 4000 more !
"Moreover many were bidden to follow me up the River in any craft they could gather" and maybe 1000 coming by the river
Without forgetting Cair Andros. It is a very strategical place, heavily guarded I think. I estimate Cair Andros Garnison at 500 men or more.
SO .... I would say Gondor, at the end of the Third Age was 15 000 mens (if you add all the soldiers I mentionned you'll find 13 000 but I think Tolkien didn't mention all Gondor forces in all places like Anorien, South Ithilien ...
I'll edit my post with other armies later, as it was already pretty hard to gather infos for Gondor
P.S : Tolfalas is a desert Island, nobody live there. Erech isn't a fortress, so no men come from here. Harondor is also a deserted land.