I think 8-12 is optimum if you’re reaching failure by that point. If you can lift something more than 12 times you’re building endurance not muscle mass. Of course, it has to be said, all of this is not scientifically backed and is hotly debated between fitness experts (if you wanna start a fight between a room full of fitness experts ask how many reps is optimal.)

But in my experience lifting heavy for 8-12 reps one day and then lifting ~80% for reps (I go for 30) after a rest day has had good results.