I used to be a numpad healer. I had every numpad key bound to something and even had 6 or so set up for "alt" usage. Probably close to 20 keys.

Worked fine.

However, I switched to healbot using repeater regions in IS and have not looked back. My healing is much better now as I don't have to remember what key goes to what toon, but simply what click casts what spell.

Highly recommend it.