A school may only sign their scholarship limit. However, their is no limit to the number of players that may be recruited. If a player commits and is offered a scholarship that is a commitment at Alabama, Georgia anywhere in the SEC. However, a recruit may back out of the agreement and a school is no longer committed even if the recruit wants to come back and join the squad.
Some marginal players will walk on without scholarship because they want to be part of the team or prove themselves at a high level rather than play for a less prestigous school.
A player may be recruited, not commit waiting and a school will often release the recruitment if they get another signee they want. Some good players are often victims of their gamesmanship by not committing until too late. Five stars always have a home. Most four stars do. Three stars need to take the best offer early at a school they desire to play for in college.
I should have added that signing is the national,letter of intent that occurs now during the early signing period in December 20-22. The next signing period is on or after Feb 7, 2019.