There is no clear answer to how many classes the school offers total, 12 seems to be the max owls anyone gets.
So how many classes you can take without needing a time tuner, would depend entirely on the current teachers class schedules.
Hermione needed a time tuner due to her classes being scheduled in the same block, but during other years those classes may have all been staggered enough for someone like Bill or Crouch Jr. to take 12 classes without a Time Tuner.
I’ve been turning it back so I could do
hours over again, that’s how I’ve been doing several lessons at
once, see? - Prisoner of Azkaban
On top of that Bill and Crouch Jr. could have had Time-Tuners anyway, but since their forbidden to tell anyone about it we wouldn't know.