If it's a sitcom or a network drama that focuses on a case of the week sort of thing, they can usually last forever. But if it's a show focused on a specific story-line, then I prefer fewer seasons. The Good Place only had 4, Breaking Bad had 5, and Fleabag famously only has 2 after Phoebe Waller-Bridge had to be convinced to have a second season, which ended up being better than the first, and nicely closed the story. The perfect amount of seasons are when the show runners know when to end a story.