Increasing the number of active motor units within a skeletal muscle is called

Motor unit recruitment.

Explanation:

Motor unit recruitment may be defined as the  addition of motor units to increase the contraction strength in a muscle. Motor unit consists of muscle fibers and motor neuron.

The single motor unit activates the weak muscle contraction. The increase in the number of active motor units within a skeletal muscle increases the muscle contraction power.  

as many times as you want

as in mitosis, dna replication occurs during interphase.   in meiosis, it occurs specifically in s phase of interphase 1.