For reference, the fifth debate in 2011 had 9 (they let Johnson in). Most of the others had 8.
The first three debates in the 2008 GOP primary season each had 10 participants.
I don't think they'll go higher than 10. Priebus has already talked about including a polling cutoff for inclusion in the debates (even if it's very low, like 1%). I doubt folks like Ehrlich will end up getting invites.
Also, the first debate isn't until August this time, possibly after the Iowa straw poll. Some of these folks will drop out by then.