Muscle grows through the activation of satellite cells, which are muscle stem cells that reside within the muscle fibers; when stimulated by exercise or injury, these cells proliferate, differentiate into myoblasts, and then fuse with existing muscle fibers, adding new nuclei and contributing to muscle growth (hypertrophy) by increasing the fiber size and overall muscle mass.
So these "satellite cells" essentially
become part of the existing muscle cell. So I think it's a bit of semantics in that I guess it depends on whether you think a stem cell that begins to grow and proliferate in direct response to training or gear is new muscle cells. Then there the idea of a muscle cell vs a muscle fiber, which is essentially a "type" of muscle cell and then myofibrils and stuff that gets beyond my knowledge.
I don't have my copy from 30 years ago but if anyone finds Duchaines Underground Steroid Handbook 2 I remember he gives a very detailed version of how muscle grows, If anyone has a copy he gets into the whole scientific process but everyone just skipped to the red pages.
I linked an article showing that elite powerlifters had more muscle fibers than controls but it could be all genetic, I just scanned the article. Thanks for an interesting discussion on the topic
https://link.springer.com/article/10.1007/BF00421171#:~:text=Despite%20large%20differences%20in%20elbow,fibres%20than%20the%20controls%20did.
