You will need to contact customer support and reference this Knowledgebase Article to have them turn on a site setting for you.

Once that is done, you will be able to make the following change:
  1. Go to Fundraising -> TeamRaiser.
  2. Find and Edit the event.
  3. Go to Step 4, Select Team Options.
  4. Go to Step 4, Multiple Captains on a Team.
  5. Edit the number and click Next or Save.