Prospective student-athletes are allowed a total of five official visits to Div I schools. And a PSA can only take one visit per school and spend a total of 48 hours of "on campus" time during the official visit. These official visits may be taken only during the senior year unlike unofficial visits which can be scheduled during the freshman thru junior year of high school.
In Div II, the PSA may take unlimited official visits unlike Div I.
With only five visits allowed in Div I it's a good idea to really research what programs to visit. And schedule the visits in order of interest (from least interest to most interest). This will give the student-athlete some trial runs on creating a good impression with coaches.