I would probably pick my top 5, as another poster said, and make the e-mail more specialized to them. Say that you are interested in volunteering to help out in the lab and want to know if you can stop by to chat more, see the lab, meet other lab members. You also want to know what your role in the lab will be (will you have a project? or just making solutions?). Free labor is nice, but it's really not free for the PI because he/she will have to train you (or make a graduate student, research associate, etc.) and hope you don't flake out in the end. We've been there. It happens about 80% of the time when we get an undergrad. So as much as PIs DO love free labor, they hate training people who work for 2 months and leave. Or who work a year but show up once every 2 weeks.
I would ultimately make my decision (because you will have choices!) based upon the 1) how it suits your research goals and 2) lab personnel and PI quality. Trust me--you DON'T want to be stuck in a lab with a bunch of people you hate, so you should meet them before you commit. Do NOT necessarily pick based on the research you think is most interesting. This is a fatal error that most students make lol, as crazy as it sounds. It will get boring at times (this is coming from 6+ years of research experience), but if you are in a good lab, they will keep you motivated and interested (as will a good PI). I picked my lab for doctoral research based on two factors:
1) Avg graduation time for students.
2) How much I liked the PI. <-This was the most important criteria in the end, by far. I'm beating a dead horse lol but really--you and PI need to be compatible.