How many synapses are crossed in a single reflex arc during a muscle stretch reflex?

One synapse. In stretch reflex muscle contraction occurs in response to the stretching within muscle. Monosynaptic reflex is the one that provide the action for the skeletal muscles. If the muscles lengthens, the activity of the nerve of muscle spindle increases. This will lead into an increase in alpha motor neuron functions. This will result into muscle fibers to contratc and counters the stretch. Gamma neurons are the one that controls the sensitivity of the stretch reflex.