Last year the Texans kept 4 RB's (Davis, Hollings, Mack, Wells) and 2 FB's (Norris, Comella), so it is pretty safe to assume they will keep at least 5 total this year.
Davis, Hollings, and Norris seem to be locks. There could be two separate battles for roster spots, both involving Baxter. One would be for the second fullback position between Baxter and Stackhouse. The other would be for the short yardage specialist between Baxter and Wells.
Offseason conditioning and special teams participation will be very important in determining who stays and who gets cut. If someone comes to camp slightly out of shape or gets nicked up during camp and misses valuable time he will find himself without a job. Also, backup RB's and FB's are required to be major contributors on special teams, so if someone can stand out as a blocker and tackler in return and coverage units he can wrap up a spot.
Competition for spots is a good thing.