After some intensive reading I found this:
Chapter: Helm's Deep
The Host passed through the breach and halted on the sloping sward above. They now learned to their joy that Erkenbrand had left many men to hold Helm's Gate, and more had since escaped thither.
'Maybe, we have a thousand fit to fight on foot,' said Gamling, an old man, the leader of those that watched the Dike. 'But most of them have seen too many winters, as I have, or too few, as my son's son here.'
That is how many were in Helm's Deep before Théoden arrived, but not how many riders Théoden had.
Isengard was at least 10000 strong, as witnessed by Merry and Pippin:
'He emptied Isengard. I saw the enemy go: endless lines of marching Orcs [uruk-hai]; and troops of them mounted on great wolves. And there were battalions of Men, too. Many of them carried torches, and in the flare I could see their faces. Most of them were ordinary men, rather tall and dark-haired, and grim but not particularly evil-looking. But there were some other that were horrible: man-high, but with goblin-faces, sallow, leering, squint-eyed. Do you know, they reminded me of that Southener at Bree; only he was not so obviously orc-like as most of these were.'
Then the Gondorian army to support Minas Tirith:
'Forlong!' men shouted. 'True heart, true friend! Forlong!' But when the [axe-]men of Lossarnach had passed they muttered: 'So few! Two hundreds, what are they? We hoped for ten times the number.'
and, a bit later:
The men of Ringló Vale behind the son of their lord, Dervorin striding on foot: three hundreds. From the uplands of Morthond, the great Blackroot Vale, tall Duinhir with his sons, Duilin and Derufin, and five hundred bowmen. From the Anfalas, the Langstrand far away, a long line of men[c. 900] of many sorts, hunters and herdsmen and men of little villages, scantily equipped, save for the household of Golasgil their lord. From Lamedon, a few grim hillmen without a captain. Fisher-folk of the Ethir, some hundred or more spared from the ships. Hirluin the fair of the Green Hills from Pinnath Gelin with three hundreds of gallant green-clad men. And last and proudest, Imrahil, Prince of Dol Amroth, kinsman of the Lord, with gilded banners bearing his token of the Ship and the Silver Swan, and a company of knights in full harness riding grey horses; and behind them seven hundreds of men at arms, tall as lords, grey-eyed, dark-haired, singing as they came. And that was all, less than three thousands full told.
He doesn't write how many were in Minas Tirith in the first place.
Then the Rohirrim come with 6000 riders, as observed by Ghân-buri-Ghân in 'The Ride of the Rohirrim'. They are heavily outnumbered by Orcs in that region.
There are roughly three times as many (18000) Haradrim as Rohirrim:
And if the Rohirrim were at their onset thrice outnumbered by the Haradrim alone, soon their case became worse; ...
The fleet of Umbar encompassed "50 great ships, smaller vessels beyond count"
After the Battle of the Pellenor Fields, the force to attack the Morannon was 7000 strong:
This then was the end of the debate of the lords; that they should set forth on the second morning from that day with seven thousands, if these may be found; and the great part of this force should be on foot, because of the evil lands into which they would go. Aragorn should find some two thousands of those that he had gathered to him in the South; but Imrahil should find three and a half thousands; and Éomer five hundreds of the Rohirrim who were unhorsed but themselves warworthy, and he himself should lead five hundreds of his best Riders on horse; and another company of five hundred horse there should be, among which there should ride the sons of Elrond with the Dúnedain and the knights of Dol Amroth: all told six thousand foot and a thousand horse.
And then, not going:
But the main strength of the Rohirrim that remained horsed and able to fight, some three thousand under Elfhelm, should waylay the West Road against the enemy that was in Anórien.
In the End less than 6000 actually arrive at the Black Gates, because some stayed at the crossroads, and some went to free Cair Andros
I'll write more if I find anything . . .