If Rubio runs, he will absolutely have a following that puts him in that group. Also, my assumption is that Huckabee will basically be running a jacked-up version of Santorum's campaign from last time. So it's really a big five or big six depending on Rubio's decision.
I'd be hesitant to include Huckabee simply because I can't see the establishment letting him get the nomination. Even if he caught fire, I think they'd be more likely to try to engineer a brokered convention somehow than let Huckabee get it. As for Rubio, I doubt there's room for two "illegal loving" Floridians in the Republican primary, and every single day Jeb soaks up more of Rubio's would-be contributors.
A majority of the delegates are pledged, meaning they are required to vote a certain way at the convention even if they support someone else. Even if every superdelegate voted against Huck, Huck could still get through easily with only pledged delegates, Romney got something like 1450 pledged delegates and the number of delegate votes to win the nomination was only 1144 in 2012. (The rest of Romney's 2061 delegate votes came from superdelegates and Gingrich/ Santorum delegates that were released from their pledges by Gingrich/Santorum) I highly doubt the RNC would be unethical enough to change the rules right in the middle of the primary season.