The average vagina is about eight inches deep. The thing is they can stretch, obviously, for taking dicks and making babies. So the problem is that the drow vagina would close over itself again unless there's a container in there to maintain structural integrity to prevent crushing the spiders. And yesterday I saw a lady with a wine bottle in her vagina. So, circumference of a wine bottle, eight inches long. As many spiders as you can fit in there. With uterus. Removing the uterus would give about another two or three inches. The issue comes down to storage now. The spiders have to have enough room to survive for their tenure of their inhabitation in said vagina bottle. Short term, you can store more spiders. Long term, they need sufficient oxygen and food, which reduces the population. Without knowing what spiders the vagina in question is to disgorge we can't really answer the question of hard numbers.