After the battle with Garou in the webcomic,
Black Sperm becomes Saitama's pet in a weakened state
Can he regrow his cells and become as powerful as he was, or all the cells he lost can't be recovered?
Black Sperm certainly seems to think so, and he should be the authority on how his cells work, but only time will tell if he's right.
In Chapter 85 he initially expects to have to start over from scratch and expects to begin by getting protein from eating bugs. But he ends up being more like someone's pet and by chapter 109 he seems happy to wolf down dog food to get him started on the path of recovery.