Max out simply means you hit the top of something and cannot possibly go any higher. So, the Ambrosia eating requirement for each immortal to "Max out" a unique skill AND accomplish all of the related skill challenges of that maxed skill means they've hit level 10 (then, the skill bar disappears).
A lot of players seem to be confused with what skills can be maxed, etc depending on what other immortals have accomplished. The most important thing is you have to assess your Dynasty's situation when an immortal is ready to eat Ambrosia. Your Founder has maxed painting, guitar and photography when he's ready to eat Ambrosia. Fine. He's in the clear and will be the easiest because you'll probably only need to think about his offspring and what they've accomplished at the time the Founder is ready to eat Ambrosia to make sure there's no overlap. You would need to choose one of those 3 maxed skills of your Founder as the one to fulfill his requirement to eat Ambrosia. And he would need to accomplish all related skill challenges for that skill as well before eating Ambrosia. So, say he chooses guitar as fulfilling his unique skill requirement. From that moment on all future immortals cannot max guitar. Why? Because the Founder then would no longer have a unique maxed skill if another immortal did the same.