I don't understand the following in red.
Some people's unhealthy habit is occasionally eating one too many donuts. But most often, with people who choose transformation, it's something more than that. There are almost as many kinds of addictive habits as there are people. Yes, they include drug and alcohol problems.
Is it just saying "some people have a habit of eating too many donuts (i.e. junk food)"?
We use the phrase fairly casually. It doesn't have to mean exactly "X+1" but it's something around that figure. If a friend saw me in the street, looking a little ill and rubbing my stomach, she might say "What's wrong with you?" and I might reply "I think I might have eaten one doughnut too many". I'm not giving her an exact figure. I'm just saying that I should have stopped eating doughnuts before I did.