This (and because game takes a while to finish) is why the game was rated AD 2/10 and not just 1. This is not hard by any measure, but will require you to play reasonably fast. The number of customers per level is not fixed, it's actually limited by your ability to make room for more customers before the store closes. Please refer to the roadmap for tips about the game but, in a nutshell, upgrade your salon, play as fast as you can, and always focus on clearing space on the waiting area. If you finished the game already and want a level to try, I'd suggest the very last one. But to me it came naturally on the 5th salon, while I was just trying to get high scores.