Both

Heaviest weight that you can handle without losing your form

Number of reps is not the main concern

For muscle growth its about muscle under tension

You can get similar muscle growth doing low reps or high reps, as long as you get the tension going on your muscle.

Weight lifters dont get huge muscle doing their heaviest lift, but through pratices reps after reps after reps.

You can grow muscle with just 1 rep, by using static hold to create that muscle tension.