The amount of force generated in an anatomic muscle for a given task is dependent on how many motor units in it are "recruited" into use. There may be hundreds or even thousands of motor units in any given anatomic muscle.
Not all of them are "in use" at any given time; if the task is one that requires little force, only a few may be "recruited" by the brain (whence, of course, all motor signals truly originate), and as these fatigue, they are shifted smoothly out of service and new motor units switched into use, to maintain the steady level of contraction that muscle has to have. In vertebrates (but not invertebrates) there is one motor end plate per myofiber and there is one axon which controls a group of motor end plates.
If there's a demand for greater force generation, more and more
units can be recruited and the total force the anatomic muscle
generates is increased. Eventually, if all the motor units are in use
and there is no reserve of rested and ready ones, the muscle shows
complete fatigue and force generation has to be halted. Feedback from
proprioceptors in among the myofibers, and at the joints, tells the
brain how much force is being exerted, how much more or less is
needed, and when to stop generating force. This is hard to learn to
coordinate, and that's why newborn babies are clumsy; they haven't
integrated their nervous system control over skeletal muscle motor
units yet. When they do, they begin to walk. When they really have it
figured out, they begin to run!