What about the Forsworn?I say they have at least 5000 also .Also shouldn't the Dominion&Thalmor have much bigger numbers since they managed to destroy the Empire so easily?Or is it because they have better weapons+magic?
They never destroyed the Empire, or even caused it to surrender. The Empire, although devestated early on, managed to come back and fight the Dominion to a stalemate (hence the WGC). And from what I understand, the Dominion's early success is mostly attributed to sudden and overwhelming attacks that the Empire wasn't ready for, though I could be wrong.
It depends on how many people are in a Legion? If it is organised like the Roman legion, it would be roughly 5000 strong.
It more than likely isn't organized like the Romand legion, especially considering how the ranks aren't even the same. I would say the Empire's legion is anywhere between 5 and 10 thousand strong, depending on how many legions the Empire has. I would say more so towards 10,000 (or even higher), considering how they only deployed a single legion to Skyrim (which isn't exactly the smallest region).