I'd have to research this issue carefully and understand the situation in this particular resource room, but I don't believe there is a set number under the law. It is more an analysis of the needs of the classroom and the students. For example, if you have severely autistic kids, you may need an aide at 2 or 3 students; if you have students with only mild needs, you may not need an aide until the class gets to 5 or 6. So it depends.
You should consult with an attorney to assure that the school district has put in place the appropriate resources for the kids.