What will be a higher number?

[Panda Express]

For reference, the first "real" debate in 2008 had 7 people on stage.

[ShamDam]

Days of Christmas, for sure. 2012 Republican debates never exceeded 9 people.

[Grumpier Than Thou]

Write-in: gaffes made by Republican candidates in the first debate

[Panda Express]

Bush
Santorum
Fiona
Perry
Jindal
Walker
Christie
Paul
Cruz
Carson
Rubio

That's 11 right there and that excludes many other possible candidates who have been making rumblings including

Huckabee
Graham
Pataki
King
Kasich
Bacchman
Blackburn
Romney
Trump
Ryan

[Likely Voter]

The peak number may not be at the first. The first debate in 2012 had five candidates but there were seven debates with eight candidates and one debate with nine. That being said, it is hard to get to 13. I could imagine 13 declared candidates but it is likely that the debate rules (especially if the GOP is in charge) will keep some off the stage (like they did with McCotter and Johnson in 2012)

[Mr. Morden]

For reference, the first "real" debate in 2008 had 7 people on stage.

Actually, it had 10:

http://en.wikipedia.org/wiki/Republican_Party_presidential_debates,_2008#Attendance

Days of Christmas, for sure. 2012 Republican debates never exceeded 9 people.

Not true.  Like I said, several of the 2007/2008 debates had 10 people.  I don't think they've ever gone higher than that.

[Xing]

There will probably be more than 12 Republicans who will declare their candidacy in 2015, but I don't think there will be a debate with more than 12, since some of them will drop out due to gaffes and such.

[IceSpear]

There will probably be more than 12 Republicans who will declare their candidacy in 2015, but I don't think there will be a debate with more than 12, since some of them will drop out due to gaffes and such.

This. Plus a few may be excluded if they poll too low.