And in those 10 years I can say that I may have learned a thing or twelve.
What's the meaning of "learned a thing or twelve"? Is it an idiom?
The phrase learned a thing or twelve isn't an idiom.
A thing or two is a common way of saying one or two things (or less precisely a few things).
The author has tweaked that phrase, replacing two with twelve. This not only increases the meaning from a few things to several things or many things, but by altering a more common phrase, it draws attention to the increase by differing from what a reader familiar with the original phrase might have anticipated.
So to sum up, you could reword your original sentence as:
A thing or two is an an idiom for a few things. "A thing or twelve" implies ignorance or cluelessness beforehand. An example would be you talked to someone about a topic that you thought you knew pretty well, only to learn a lot of stuff from them and realize the you were really a lot more ignorant of the topic than you had thought. Afterword, you might say "I talked to Bill about Thatcher's economic policies and learned a thing or twelve."