The number of participants is directly related to the number of projects you bring on. A good rough estimate of is 10-12 people per project. This gives a little buffer for people that don’t show up or can’t stay all weekend. After that you may find teams split up, but invariably it works out- but more on that later. Begin talking to project owners such that you may have an extra project in case someone has to back out.
Here is a more thorough guide on choosing projects: Making a Successful Social-Good Hackathon: Crafting Projects for Success