The maximum number of participants is determined by the type of conference bridge that you are using. If you are using the built-in software conference bridge, it can support up to 128 streams which could be 128 users in a single conference or 42 conferences with 3 users each. Different hardware conference bridges support different numbers of conferences and participants. Take a look at the following link.

http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00803edae2.html#wp1035397

Hope this helps. If so, please rate the post.

Brandon