Oh, what a good question.

There have been a couple of good answers on this topic already, which is a pity because I was hoping to write another! Here's one 8 years ago from u/crow_hill, and another more recent one from u/toldinstone, which is excellent and well worth the read and the follow ups (which include an image of a sample ticket from a different amphitheatre).

I'd add to the bibliography of these answers with P. Rose (2005) 'Spectators and Spectator Comfort in Roman Entertainment Buildings: A Study in Functional Design', Papers of the British School at Rome, 73:99-130 [Jstor]. It's a top notch study of the design of amphitheatres, and how they worked for the people who used the seating.