It doesn’t sound like the 49ers are planning on outright resting many starters vs. the Rams. In fact, their injury situation is such that they can’t rest more than a couple players in order to field a full team.
The more likely scenario than players simply sitting out the game is that most starters will begin the game and then get removed at a certain point to make way for backups, a la the way the third preseason game is typically handled.
With some starters either injured, sitting out, or possibly taking on a more limited role, it should open the door for a handful of players to get an extended opportunity: