For mod I like 9 or 10 doubles with rollout around 9.37-9.4 depending on amp draw and at least 430 run time on your batteries. That rollout is for motors drawing about 5-6 amps. Play with your spring tension to bring the amp draw down. You don't need much tension for 4 cell on carpet.
My best motor now is a 9x2 Ti and it only draws 4.8 amps. Not sure why it's so low, everything else is the same as my other motors. It's just one of those REALLY good armatures from Team Br00d.
take care
john