I started out with 3 Thirsties Duo Size 1 covers, and then I got 2 more second-hand three months after DS was born. So I felt that I needed those 2 extra, but I survived for a while with just the 3. It was just one of those things, sometimes it'd be fine and then other times poop would get on the cover every single time and I would have to wash a little sooner than I would have otherwise needed to. (I was washing every other day. So I guess I didn't want to handwash the covers separately.) If your baby ends up pooping a lot on the covers then you may decide to wash each cover individually as it gets dirty, or you might be able to do two at a time. They dry relatively quickly, especially if you hang them somewhere with good airflow.
I have two OS Flip covers also. I can't remember how often I used them in the very early days. I think I used them at night, maybe, not sure when I started doing that. They may or may not fit very well to start with. DS was almost 9lb at birth, and they did fit him but I definitely had to pin/snappie the diaper because else it would sag and he could pee out the side :B Either way, it should work if you have to use them in a catastrophe