It seems like some people have more muscles cells than others because they gain and retain mass more easily. You don’t grow new muscles after you are born, but it seems like some people are born with more of them.
Of course you grow more muscle cells after you are born. You might notice this through adults being larger than children.
The only type of cells I can think of where the number is fixed at birth is primary oocites in frmales.
OP, there are numerous reasons some people get faster results from resistance traing. It’s a pretty long list.
Too late to edit and add:
It may be the case that muscle hyperplasia occurs in certain conditions, but it’s not an “of course” issue; we get bigger muscles without it.
Okay, so color me suprised.
There is one genetic factor that may be of interest. Myostatin is a protein that inhibits muscle growth and fat deposition. Some animals have a genetic mutation that limits myostatin production, and so they tend to be more lean and muscular than their peers. Belgian Blue cattle have been bred for this trait, along with “bully whippets,” of which Wendy the Whippet was probably the most famous. This mutation happens in people too. Liam Hoekstra comes to mind.
Machine Elf, I was actually going to use myostatin deficiency as an example of one of the more rare reasons some gain muscle more easily. Flex Wheeler is said to have it.