Harriet, is teaching a particularly low paying profession? In Australia, the starting salary for a graduate is close to $60,000.
I think so.
My mother is a teacher and within five years of retirement (though she's inclined to work longer because of her desire for a comfortable retirement income). She is OK working just that job, but she does tutor in the evenings and in the summer so that she's not living so close to the edge. Many of her peers do the same, or other part-time/summer work.
When she started full-time teaching after a few years away and was the only income in a family of five, we qualified for the reduced lunch price program in our public schools because her income was so low.
When I was in school, I typically had 2 part-time jobs, and I did the same right after graduating until one job went full-time. For a few years, I've supplemented my income with freelance work, which is like a second job (or my only job, now that I'm in school again).
There's a varying threshold above which people are comfortable, but below that, it's reasonably common to find additional work until you earn enough to make ends meet. I would not speculate about percentages or numbers of jobs, though!
I'll just say that if I find out a person is making minimum wage at one job, I'm never surprised
to find out they're working a second gig, too.