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I've used as many as 3 in my Motif ES.

You can operate in one of two global modes. 

The first (default) is that each card is usable as a separate 5-voice synth (for sequencing, splits or layers). This is how I used them most of the time.

The second is "poly expand" where the cards act as a single synth with 10 or 15 voices, all playing the same patch (the cards to have the same user patches loaded).

If you have more specific questions, let 'em rip.