Bloom Dragons are something I always try to stock up, even when I don't have any girls in immediate need of evolving.
Earlier in the game, I'd advise to keep at least 3 of each lvl 100 Bloom Dragon on stock. Whenever you go below, run the weekly map religiously until you have at least 3 of them again. Later on in the game, as your inventory size increases, you can increase your minimal stock to 5 or 6. For the lvl 20 and lvl 5 Bloom Dragons, my minimal stock size is as much as 15, though I finished evolving my 2* girls, so I could probably lower that amount again, at least for the lvl 5 Bloom Dragons.

Keeping Bloom Dragons on stock saves me from a LOT of frustration when I want to evolve a girl. Especially when I have one of those super rare 6* girls, which I generally want to level, evolve and max as quickly as possible.