it can depend on your baby's shape, as some brands have a wider crotch fit than others. might be a good idea to get a couple of a few different brands until you work out whats going to work for you.

Some people do not wash the covers after every use, they change the insert and liner/doubler but leave the same cover on. Others change both every time.
Also with DS i am sure I have pockets that werent AIO and needed a cover on top as well - for those i could re-use the cover and wash the pocket and insert.