John, 18 is fine if you choose 17.5. For 13.5 it's a recipe for a pile of broken bits and not a little frustration as teams try to put on the laps. I'd also be a bit wary of the crowd in pit lane, and the potential for 18 marshals running around the track. IMHO, 16 is OK, but 18 may just be too many. HTH
