500 seems to be about 90% full (see attached). I say "seems" as I'm not sure what the range is on the fill there (ie, is the "0 line" at the tip of the cervix/colon, or at the bottom of the crescent UI element?).
As for how much intercourse it would take...that might be a complicated question. The feeling I got from Syahata's was that each creampie has a slightly randomized amount of cum in it (going from memory, *roughly* 30 for humanoids and 50 for certain special enemies). Some quick testing suggests a comparable situation here (as could be expected), with futa nurses doing ~30-40 per creampie (30.7, 39.7, 30.5), mosquitoes doing ~45-50 (47.4, 49.7, 46.5 - probably ~40-50 and they just rolled really consistently. Also, assuming that green and brown have no differences, since they seem palette swaps atm and anal isn't my thing so I don't care to keep testing), and Ol' Doubledicks doing ~30-40 x2 (37.5 / 34.4, 37.1 / 33.1, 30.5 / 31.7; again, assuming 30-40 and the anal creampie just rolled consistently).
Note that this was a quick, not particularly rigorous test, so I might be missing something here and this information might fall out of date pretty quick depending on how the game develops.