Can I take courses in my concentration for "S"? Is there a limit?
YES, you can.
BUT, that said, it is wise to use good judgment in this regard. If you are thinking of applying to medical school, or other professional or graduate schools, admissions committees will want to see evidence of excellence in science and other courses. So, you will need to consider when and if and how often the "S" vs. grade option is a good idea.
If a course is offered "mandatory S," this fact is indicated on the transcript, so it is out of your hands.
In addition, if you transfer courses to Brown from study away programs, they will ALWAYS transfer with grades of "S." If you received grades from other institution for such courses, you can request transcripts from there to go along with your Brown transcript in the future. These actual grades can be used in computing an "external GPA."