You can still back count. If you sign under 25 in the previous class you can back count with players who enroll early.
The second rule: An FBS team can bring in 25 new scholarship players in one signing class.
The limit is on 25 initial counters, which you can think of as players in their first year on an athletic scholarship. But a team can bring in more than 25 enrollees in a given year, in a few circumstances. Most commonly, if an athlete graduates high school early and enrolls for the spring semester, and if the school stayed under a cap of 25 in the previous year, it can assign enrollees to the previous class, up to the 25-man total cap.