How many sparring partners should I choose for the most effective motivation in the gym?
The Ringelmann effect may be useful to you in this question; he found that "the addition of co-workers in a rope-pulling task leads to a linear decrement in the individual group member's average performance."  It has been examined experimentally here, and was found to occur with the addition of second and third group members, but adding members beyond that didn't have a significant effect.  There was a second experiment that tried to control for the possibility of intermember coordination, but it found the same thing.

(Of course, this leaves out entirely the possibility of differing group dynamics across groups of the same size but with different members. It also does not speak to individual differences--that some people may do better with a larger group, or with varied activity partners, while others do better with smaller groups or more consistent activity partners.)

Alan G Ingham, George Levinger, James Graves, Vaughn Peckham
The Ringelmann effect: Studies of group size and group performance
Journal of Experimental Social Psychology, Volume 10, Issue 4, July 1974, Pages 371–384