Is it a risk working for a graduate advisor who has over 20 students in their lab? I recently contacted a professor whose field of study is something that I really want to work on. He replied, that frankly his lab was pretty full at the moment, but he'd love to hear about my plans for graduate school. Since I have already secured funding, I guess this does not automatically disqualify me from working in his lab. My only concern is that in such a large group of people, and not as a student working off of funds the professor secured, I would not get the kind of mentorship I would get in a smaller lab. Just looking for any insight that could help me in making some choices in the future.
Being part of a big lab has pluses and minuses, and whether it's a good thing for you is likely to depend much on your personality and circumstances.


Cons: you've put your finger on the key fact, that students in big labs are likely to receive much less attention that students in small labs.  You also might end up as a just small part of some big project, or even just a glorified lab tech.
Pros: in order to support so many students, a professor generally needs to have good funding and being doing research that lots of people find exciting.  This means that the projects you are working on are likely to have people paying attention to them, and that if you do well, you may have a powerful backer.


For somebody who thrives in a big, competitive environment, a big lab may be good.  For somebody who wants a more personal relationship with one-on-one tutorial, a small lab is probably better.  All sorts of labs and advisors can be routes to good careers, however, and it's a matter of figuring out what works best for you.