I would say 6 to 10 covers is plenty. I don't change the covers everytime, only if they feel wet or are soiled. 6 would be enough for DS1 but I go through more with DS2 as his BF poos leak out of the flats more often.
Covers can be thrown in the wash with normal clothes so you can do them anytime, you don't have to wait till you have a full load of nappies. Actually I avoid putting mine in with the nappies, otherwise they are just covered in fluff.
:yeahthat:

We used cotton fitteds for the first 12 months, with PUL covers, and we had about 5 or 6 smalls and 6 mediums. You *may* need more for flats though? The fitteds kept the poo in nicely, we rarely had leaks onto the cover, so we only needed changed the cover after a few nappy changes, when we felt it getting damp around the leg elastic.