Is there a limit to the number of participants?

Nope. Challenge Hound can handle as many participants as you can convince to join your challenges. We have however found that challenges are often most engaging when the groups are tighter and have more in common. If you are expecting a large group, maybe consider advanced, intermediate, and beginner challenges. You can also split out challenges by activity type, possibly a run, ride, and multi-sport challenge. Participants are more engaged when they feel connected with other participants.