There is no perfect number - use common sense
There is not perfect number of colleges to apply to. I have students who apply to only one because that is the school they want to attend and are admissible. I have students who apply to multiple schools because they have an idea and want options. We tell our students to consider two reach schools, two 50/50 schools and two schools they know they will gain admission. This is different for every student. One thing to consider when applying to multiple schools are the application fees. Many colleges charge anywhere from $25 to $75 in application fees and do not waive them no matter what application you use. Keep your numbers reasonable. Do your research first, narrow your choices and then apply.