Team can have up-to 48 players active but I believe two of them must come from the practice squad. So if all rostered players are healthy, 7 must sit on game day and 46 can play.
Here are the 7 I think are likely to sit if no one is injured:
1. Kyle Allen - A QB will sit for all games. I believe it will usually be Allen and not Heinicke.
2. Sammie Reyes - This is a developmental year for him; in most situations (if at least 51 players are healthy), he will sit unless the injured player is a TE.
3. Mayo or Forrest - Forrest is on the IR now; replaced by Mayo. I think either one of them is likely to sit (if safeties and / or LBs are healthy) as Jared Norris could replace either one on special teams from the practice squad.
4. Toney or Toohill - One of these two players will sit each game if Young, Sweat & JSW are healthy; it may depend on if special teams or the d-line is expected to be the larger role.
5. Roberts or McTyer - I expect one of these two players will sit each game if other CBs are healthy
6. Milne or Carter - I expect Milne to sit unless he's become as dependable as Carter as a punt returner or if a number of the injured players are WRs
7. If they need a 7th healthy player to sit, my guess is that it will be Toney or Toohill, the one that is not already slated to be inactive for the game.
Sometimes the selection of who to sit could depend on who from the practice squad should be called up / least amount of drop off;... similar to the example of Norris above.