During the peak season, all of the ride vehicles are generally available, with some exceptions. There is an ongoing annual inspection process where every ride vehicle is dismantled to the bare frame, throughly inspected, hardware replaced an reassembled. It happens to every vehicle, even 80,000 lb. Kong trams are removed and processed.
The compliment of spares, which includes coaster cars, allows for much of this to go on year round. That's what goes on in most attactions, including Spiderman. In some cases, the process takes place only in the off season, depending on the attraction. This is an annual process that repeats every year.
On a day to day basis, or time of day basis, the number of vehicles on the ride is dependent on the guest load. Typically it peaks up mid-morning and tapers off as the afternoon goes on. When foul weather approaches, the number of ride vehicles is reduced on outdoor attractions to reduce the guest exposure to the bad weather and improve evacuation time if necessary.
I hope that answers the question.