Glad I try to maintain a supply of 3 lvl 100 dragons and 5 each of the lvl 20 and lvl 5 dragons of each color at all times (so yes, that's 52 slots for dragons!), even when I don't have a girl lined up to make use of them.
Although, I did have to expand my girl capacity to 150 to keep that managable, with room to spare for Manyus and running normal gacha multiple times before vendoring.
I just figured I'd need those slots eventually anyway. By the time I evolved all 2* and 3* girls and can stop collecting lvl 5 dragons, I'll have just the same spare room again..