Per US Today article how 105 roster works

College football teams must submit a 105-player roster to the College Athlete Payment System 24 hours before their first game, but limited exceptions allow replacements for injuries, suspensions, withdrawals, and designated student-athletes who do not count against the cap.
Key points:All FBS teams must file their rosters with CAPS no later than 24 hours before their season opener.
CAPS operates under the oversight of the College Sports Commission.
A submitted roster of 105 players remains fixed for the season unless specific exceptions apply.

Teams can add players up to the 105-player limit if they did not initially submit a full roster.

Injured or ill players, permanently suspended players, and students who fully withdraw from the institution can be replaced on the roster.