No. You can strengthen muscle without increasing the number or mass of muscle fibers. That is typically done by lifting a heavier weight for a few reps. If you want to build muscle mass, then typically you lift a lighter weight for a lot of reps. IIRC of course.

Not all of a muscle is used when performing it's function some of it is kept in reserve. Lifting a heavy weight a few times trains that muscle to recruit more of the fibers it already has for the lift. .

Lifting a weight you can do for many reps will have the body increasing the muscle mass but not necessarily increase the weight one can lift.