...An area I've studied : )
I assume you're talking about Herodotus' The Histories.
Well it was a few hundred/thousands but while the book recorded numbers in the millions it is far more likely to be dealing with an army in the size order of 100,000-200,000 or so although don't quote me on that as I can't quite remember the numbers that classists worked out.
However, the reasosn why ths Spartans did so well was because of their superior, training, discipline but also armour and weaponry. Alot of the reason they managed to hold the choke point was because their spears were longers than the Persians while the Persians could not move round them because it was a choke point. Of course the Spartans holding them was only possible because of their training and discipline.
Either way, as you originally point out - absolutely amazing. That they fought against an army that size, had them beat them for a couple of days despite the fact they knew they were facing inevitable death.... astounding.