We don't have a problem finding the guys. We play with 4 other couples, so it always involves them. In our little group, it is almost anything goes (no toilet, pain, etc), so everybody gets ganged at one time or another. It is a bi group, so the guys get ganged too.
Some people talk about having very large groups for a gang session. That might be overwhelming. Ali says any more than the 5 guys in our group would be more about me satisfying my own voyeuristic interests than her sexual desires. Also, the number might be determined by the activity. If it is an anal GB, you may want fewer guys. She doesn't like more than 3 for anal GB. She has done 5-guy facials but thinks 2-4 is the best number to enjoy.
We prefer to GB with our own little group. We think it is safer and more enjoyable. Plus, we can do bareback with them. The other couples feel the same way. We recommend couples considering GB, try to form a close-knit group, where everyone enjoys each other. It is an awesome experience, with the right group.