Very interesting idea. Please help me understand is there is a specific business case behind this or if you were just curious. I don't know of a way to do this out of the box other than to:
1. Restrict the date range the activity is running for in order to limit the number of visitors.
or more exact.
2. Manually de-activate the activity once it hits your goal number of visitors.
Mihnea Docea | Technical Support Rep, Customer Care | Customer & Employee Experience | Adobe | (:: 1 (800) 497-0335