How to ask a professor to be your thesis supervisor?

This isn't a situation where a mass email is appropriate. You'll be working with this person for a year, so you want to make sure you can get along and you're interested in the research they do. They'll be devoting a large amount of time to you and your project, and they do NOT get paid to do that. You're asking them to give up a lot of time. You need to talk to them about this in person, and not approach many of them at once. One at a time, and if you get turned down, try another professor. They will want to see your resume, and hopefully they've had you as a student at some point.