What I have experienced is compatible with what I have heard from others: University admissions is very good at adjusting for the rigor and grade scale used in different high schools. As such what high school you are coming from really is not going to make much difference for university admissions. I can give you two examples, but I have seen more.
One example: I came from a high school where no one in the entire high school had straight A’s, or even anything remotely close to straight A’s. There was only one student in the entire high school who had an overall average of 90 or better. The number 2 student in the high school graduated with an 89.9 average (which is sort of easy to remember since he missed “90” by the smallest possible margin). This did not hurt us in university admissions. Two students went to MIT. One went to a top school in Switzerland (possibly EPFL Lausanne). Several others went to McGill. If we have gotten the same grades at a normal high school in the US, we would not have gotten into these schools. However, admissions adjusted our high school results based on the grade scale used in the high school that we attended.
Similarly as a freshman at MIT I met a lot of students who were either number 1 or number 2 in their high school. I met one freshman who had only been number 7 in their high school. However, they had attended a famous high school that is known for math and science and academic rigor. MIT admissions interpreted their high school results based on the high school that they came from. Being 7th in a normal high school would not have gotten him into MIT. However, if he had attended a normal high school he probably would have been ranked higher than 7th, and likely still would have gotten into MIT.
I do not think that coming from a highly competitive magnet high school is going to make much difference one way or another in terms of helping or hurting your admissions to university. However, it might have two effects. You might be surrounded by students who expect to to attend famous universities. Ignore them. Look for a school that is a good fit for you. Also, your grades might not look quite as high as they would have been if you had attended a more average high school. Do not worry about it. University admissions will adjust for this.
For admissions to highly ranked or famous schools you will be competing with thousands of applicants from all over the US, and all over the world. Any competition from your particular high school will not matter much. For consideration for merit financial aid again you will again be competing with lots of students who are coming from all over the place. There are lots of colleges and universities and you should be able to find something appropriate for you.
I do not think that you should care about how many applicants from your high school apply to any particular college or university. Instead, look for colleges and universities that will be a good fit for you and that are likely to be affordable for you given whatever budget you and your family can afford.
Best wishes.