A neutral carries current in the (very) likely event of an unbalanced multiwire circuit. So, yes, the neutral counts unless you know for sure that it will have no current on it.
I hate to step on toes leland but 358.60 (2005) says that EMT shall be permitted to be an equipment grounding conductor. So no equipment grounding conductor needed (but I recommend it).
To answer the original question. The termination ratings have nothing to do with the derating value of the conductors, only the overcurrent device used. So you would use the 90C (thhn) column of T310.16 for derating as necassary for T310.15(B)(2)(A).
My Ugly's says 16 #12s in a 3/4" EMT. If you want to keep a 20 amp breaker on those #12s then you would be limited to 9 current carrying (thhn) conductors (30x70%). This includes your neutrals (unless you know otherwise). So, 6 circuits (with 3 neutrals) in a single phase system, or 6 circuits (with 2 neutrals) in a three phase system. The grounding conductor (if present) does not count in the calculations.
Feel free to correct me if I am wrong
James
