i had a rotation of about 8-10 on the go when i was using flannel flats full time with DD for the first 7 or so weeks. they contained any explosions really well.

if the nappy was really wet, i'd grab a new nappy and just let the cover air for a while. if the cover felt wet, or was soiled, it got chucked in the wash. i was using Maz covers (fleece or minky) - still use them overnight every night - have about half a dozen on the go now - but am only using one most nights - only end up using a second one if DD has an explosive poop...