It’s All About the Volume
Optimal training frequency is determined by optimal volume, not the other way around.
Sometimes when you’re trying to improve your understanding of a concept, it helps to imagine the extremes, so let’s apply that to training frequency.
If I were to suggest that you should only train once a week, you’d probably (and rightly) doubt my credibility, because that workout would probably take 4-6 hours, at least if you were trying to do enough work for every muscle group. So once a week is obviously a non-starter.
Now consider the opposite extreme: Imagine training 14 times per week. I guess the upside is that each workout would be super short, allowing you to impart maximum intensity into each training session, so there’s that at least. But the obvious problem is that it hardly makes sense to drive to the gym twice per day for only a couple of sets.
Somewhere between these two extremes is the optimal training frequency, and this depends largely on how much total work you’re doing each week. Put another way, if you need to do 16 sets for hamstrings each week in order for them to grow, you probably wouldn’t train hamstrings once a week, but you also wouldn’t train them 8 times a week. It turns out that 3-4 times per week tends to be the sweet spot for most lifters.
Bottom line, it’s not frequency, but VOLUME that drives muscular growth, so simply use a frequency that makes it easiest to hit your optimal volume.