The short answer
Two to four sessions a week is enough for most people to make steady progress. What matters most is training each muscle about twice a week with enough hard sets. A 2016 meta-analysis found that training a muscle twice a week builds more muscle than once; beyond that, total weekly volume matters more than the number of sessions.
Start with what you can repeat
The best frequency is the one you can keep for months. Three sessions every week beat five sessions one week and none the next. Choose a number of sessions you can fit in even during a busy week.
Twice a week for each muscle
Schoenfeld and colleagues (2016) compared training each muscle once or several times a week with similar volume: twice a week gave more growth. You get there with three full-body sessions, or four upper/lower sessions.
Volume matters more than the number of sessions
Once each muscle is trained twice a week, extra sessions mostly spread the same work out. Ten hard sets for the chest over two or three sessions give similar results. More sessions help when they would otherwise be too long.
Examples by schedule
Two days: two full-body sessions. Three days: full body on Monday, Wednesday and Friday. Four days: upper/lower twice. Five or six days: push–pull–legs, for experienced lifters with time to recover.
Recovery is part of the plan
Muscles grow between sessions. Leave about 48 hours before training the same muscle hard again, sleep enough, and take a lighter week when fatigue builds up.
Put it into practice
About this guide
STARK editorial guide. Examples explain how to use the library and record training; they are not individual coaching.
Training principles: Schoenfeld, Ogborn & Krieger (2016), Sports Medicine.
The session examples come from the STARK program catalogue linked above.