5 toons generate mana at 5X the rate of one toon. There is no reason they will not kill 5X as many mobs as one IF you have unlimited mobs.

Same applies to 2 vs. 5 or 3 vs. 5 etc.

And the group bonus kicks in. So ASSUMING unlimited mobs you go 5 or 10 or 15 for max efficency.

Also, judging from prepared fast leveling of 40 plus toons the simple fact that if you quest more toons dosnt seem to slow you down a whole lot ...