Assuming Purdy starts the rest of the Niners' games this season, he'll have 5 games starting under center, plus any playoff games, and most of last weekend's game. That would be more starts than Lance, and almost as many total games, maybe even more if the Niners get into the playoffs and win a game.
That could make training camp interesting next summer. And SF does have some history of a starting QB losing his job due to injury. See Smith, Alex, and Montana, Joe.
Edit: I just went and looked, and of the other QBs taken in this years draft, three - Kenny Pickett, Malik Willis, and Bailey Zappe - have actually played so far. Small sample size, but Purdy's completion % is better than Willis (Rd3), and comparable to Pickett (1) and Zappe (4) and his TD/INT ratio is better than any of them. (Purdy 2/1, Zappe 5/3, Pickett 4/8)