How many space marines were in a pre-heresy legion? (warhammer 40k)?

I am reading the Horus Heresy novels, and was wondering if there were more space marines pre heresy than the 1000 that are in a chapter in the 41st millenium. I ask this because at the present time there are about 1000 chapters with 1000 marines each, but back then there were only the original 20 chapters, so were they much larger or were there only 20000 marines spread across the entire Great Crusade?