You may set a limit on the number of Spots each Participant is allowed to sign up for in the Details tab of your SignUp's Organizer view (towards the bottom of the page). This option is available with all of our Premium plans (starting at $9.99 per month).
A limit may be set per SignUp, per Day or per Week (Sunday - Saturday) - meaning that each individual Participant (identified by their unique email address) will be allowed to sign up for a maximum number of Spots given the limit time frame you specify.
If the SignUp is also set up with the option to allow people to Sign Up Others (family members, team members, etc.), then the limit will be applied per email address. For example, if the limit is 3 Spots per SignUp, a parent signing up her kids (who don't have their own email addresses) would have the ability to sign up a total of 3 times. If a coach was signing up team members that do have their own email addresses, she could sign up each person for up to 3 Spots each.
For a tour of all your Organizer options including navigating to the Details, Participants or Spots tabs, please watch the quick video in this help article: Managing your SignUps