how many communion slaves per master?

[Taqwus]

Unless you use a communion master matrix crystal, a master-to-be must cast Communion Master himself, and he won't necessarily do this unless you script him to do so.
You might consider giving your Communicants explicit 'hold' orders (hold x 5, stay behind troops) unless you're going for the Reusable Exploding Communicants of Doom tactic (!).
I second the suggestion regarding having at least one master have Nature magic and enough support to get to Nature 3, plus the research necessary for Relief, as Relief will reduce fatigue on ALL your units including communicants. Very useful for keeping them from dying due to fatigue, as is Summon Earth Power; Invulnerability is another nice spell for a master to cast.