Effective for the 2018 recruiting cycle, there is now a separate limit of 25 signings (NLI or aid agreement) per academic year. If a player signs an NLI but doesn't enroll, the school does not get that signee spot back to use for another player. Also, blueshirting can't be done anymore -- because an initial fall scholarship counts against the current academic year's signing limit. Btw, transfers (all types) do count count toward the signing limit as well as the 25/85 (initial/total) counters limits.
NCAA Bylaws:
13.9.2.3 Limitation on the Number of National Letter of Intent/Offer of Financial Aid Signings. [FBS] For the football bowl subdivision, there shall be an annual limit of 25 on the number of prospective student-athletes who may sign a National Letter of Intent or an institutional offer of financial aid and student-athletes who may sign a financial aid agreement for the first time. (Revised: 4/26/17 effective 8/1/17 for signings that occur on or after 8/1/17)
13.9.2.3.1 Application. [FBS] A prospective student-athlete who signs a National Letter of Intent (NLI) or an institutional offer of financial aid or student-athlete who signs a financial aid agreement that specifies financial aid will be initially provided in the fall term of an academic year shall count toward the annual limit on signings for that academic year. A prospective student-athlete who signs a National Letter of Intent (NLI) or an institutional offer of financial aid or student-athlete who signs a financial aid agreement that specifies financial aid will be initially provided during the second or third term of the academic year may count toward the limit for that academic year or the next academic year. (Revised: 4/26/17 effective 8/1/17 for signings that occur on or after 8/1/17)