Volunteers are able to sign up for as many Shifts as they want unless the Manager for the Opportunity has put restrictions in place. Restrictions are set by managers, and are used to limit the total number of shifts a User can work.
Restrictions can include:
Limiting the number of shifts volunteers can select by providing a minimum and maximum amount of shifts
Limiting the number of Hours volunteers can select by providing a minimum and maximum amount of Hours
Disallowing overlapping shifts
If you have any questions about the limit on Shifts for a specific Opportunity, contact the Manager for that Opportunity.