In flexibility training is one set of stretches sufficient or should I have clients repeat the set several times?

All excellent responses. Understand that there isn’t a lot of research specifically focused on stretching. One thing that is consistently shown is that in order to improve joint range of motion it’s important to stretch at the end of a workout, especially cardiovascular, so fascia affecting ROM is “warmed up” and more amenable to the stretching overload.

It appears that one set of stretches held for 10-15 seconds in a position where there’s tension (not pain) in the target tissues can be effective over time in improving ROM. 2-3 sets may be somewhat more effective.

Take care.