My high school doesn’t offer many AP courses, and this will probably affect my exam scores in the case that I self-study. Will college admissions know this, and how?
Usually this information is included in a document called the School Profile that your high school submits to colleges along with your transcript. You can probably find your high school’s School Profile document somewhere on its website under the college counseling section or ask your high school counselor.
In addition your guidance counselor is asked to rate the rigor of schedule (relative to what is offered in your HS) in his/her recommendation.
The school profile will generally list how many AP and/or honors classes are available and whether you took the most rigorous classes that were available to you.
And generally the counselor will note how the level of rigor of the classes you took compares to that of your peers.
Self studying for AP tests will not increase your rigor. Unless you are doing it to get cheap credits, many would say that your time is better spent doing something else.