the short answer
Training frequency for muscle growth is how often you train each muscle per week. For most clients, training a muscle about twice a week is the reliable default - at equal weekly volume, 2x tends to slightly edge out 1x, mostly because spreading the same hard sets across two sessions keeps each set fresher. But total weekly volume, not frequency, is the real driver of growth. The best frequency is whatever lets a client hit their volume target and recover.
This article is general programming guidance for coaches, not medical advice - individual recovery, training age, and health history vary, so adjust the dose to the client in front of you and refer medical questions to a qualified clinician.