So far i've only multiboxed with druids, dk's, pallys, mages and shamans

Enhancement shaman - 10 pve+pvp
Paladins - 10 pve+pvp
Death knights - 8 or 9 pve+pvp, depends if i play with healer or not
Mages - 10 for pve
Moonkins - 5 for pvp until i get feared and 3 after my space owls have run to different parts of azeroth
7 for pve before you know how to manage eclipse
10 for pve after you know how to manage eclipse