There is no minimum, but most formats are designed for groups of 20 to 500 people. For small groups (15–40), tournaments or team challenges are the best choice. For mid-sized groups (40–200), you can run several disciplines at the same time. Large groups (200+) benefit most from a festival setup with fixed activity stations.
Bear in mind that with mixed teams the group composition determines the feeling. Random teams break through departmental boundaries. That strengthens the bond. Deliberately composed teams can serve a specific aim, such as cross-departmental mixing or role mix.