Let's figure thousands and leave it at that. ßruce is right in that there are a lot of factors, way too many to get some sort of estimate going. Holidays, weekends, school trips, corporate events, and so much more can affect attendence, including what's new or not so new that year!
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Po!nt of View: A different look at Roller Coasters.
http://www.crosswinds.net/~justmayntz/thrills/index.html