I use Baby BeeHinds PUL wraps/covers, to go with their nappies.

Am using 5 atm, but have three in rotation at any one time. It is recommended that you air between use.
I only wash nappy stuff every second day, so I will wash 2 covers with each wash and then start using the other 3 until the next washing day IYKWIM

There is also no need to wash them straight away if you have had a really wet nappy and the cover has ended up with a huge wet patch, because this is what they are designed for.