as many as possible
if you give a master 2 slaves, he will get +1 to all magic skills, 4 is +2, 8 +3 etc. Combine that with the fatique-redistributing effect of communion and a few neat items and you will soon have mages that can cast all the neat air, astral and water spells. Against ai, try master enslave once, I promice you wont be disappointed. (also use rune smasher, spell focus and green eye (or ring of sorcery))
And for the secon question, they will be part of communion for anyone who has the spell communion master, sabbath master cast or is carrying a crystal matrix.
If you mean that will the priest benefir from it, yes it will. Once fighting ai AE Ermor I mae bartholomeus the patriarch into my prophet, gave him the sword of justice (so he is lev7 without communion) and teamed him up with 16 communion slaves (so +4, total 11). Ordered to cast quickness, communion master 3xbanish he shot right into hall of fame 1st position in his 1st battle, where he banished the entire opponent slow undead force, including the commanders, before it could reach my troops on the opposite side of the map.
Also, a very neat pythium strategy: get earth-10 blessing, rather cheap with cyclops. Both your communion slaves and masters are sacred so on first combat turn cast divine blessing and your mages are going to be heavily reinvigorated. Also, the cyclops can boost itself as high in earth magic as it can cheaply do with items, and start casting enliven statues. You get 2 more statues per path of earth magic exceeding the demands, meaning you get very cheap statues very fast. They are not the best in causing damage, but they are a wonderful screen in front (and around) your mages as they are rather slow to kill.
Re: how many communion slaves per master?
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