80 is the max for each. You can fight them as many times as you wish assuming you have enough energy, refills or units for refills.
80 is the max amount of POINTS (aka, Health Bars Cleared), over however many fights it takes you to reach that max, for EACH of the Coliseum Panels (Defenders).
But you have unlimited # of attempts (fights), with same champ or with as many different champs you want to try as well, in which to clear that many Health Bars.
If all you can clear is 1 or 2 health bars per fight, you can just keep repeating 40 to 80+ times. Or if you are able to clear 10’s of bars or more each fight you will thus need to do less # of fights.
Its a little late to be asking this question during the final few days of Solo colesium round 1. You have a crazy amount of grinding to do to complete all 4 bosses, unless you are a highly skilled pro player who can consistently clear 15+ health bars every run.
To answer your question, there is no limit on the no. of tries for each boss to clear the required 80 health bars. Each run costs 25 energy, so you should keep that in mind. Read the nodes and bring your best champ for each boss fight. Aim to clear as many health bars in each run to minimize the no. of attempts and subsequently, the time and energy cost required. You can also chip away one bar per run if you dont have the best counters for a fight. Be warned, though, it would be soul crushing this way.