to be exact, 300 spartans, 700 thespians, and 400 thebans, if youre doing the final day. but you would have to choose a way to make it, with either the first day, where mostly hordes of light infantry attacked, which would probably be easier,the second day, with immortals, cavalry, and heavy infantry fought, which would be medium, or the final day, where everyone encircled the spartans and their allies, which would be nearly unbeatable unless you have a ready made plan.also, you would need some sort of gates as there were in the actual canyon, plus some way for the greeks to shield against the countless persian arrows. i think shields in more or less a testudo-like formation would work good, and then the archers would fire until they started hitting thier own guys, and fire again once 50%, 25%, and 10% of their men were left. after 10%, they would continually fire. i think thermoylae would be difficult to recreate though. try making like plaetea(a year after thermopylae, pwnd those persian pricks)