The park's capacity at any one time is 28,000 (or was, this may have been reviewed I guess). It's based on what has been aggreed with the local authorities, and their own safety personnel. You can get more than 28,000 admissions in a day (as seen in the 2010 figures released as part of the Smiler planning application), but people do leave early/arrive late remember.
Not sure where last Saturday was in terms of numbers, but I think this Saturday will surpass it as the busiest day of the season, particularly of course as the Saturday is 5 November this year. A sellout is not beyond the realms of possibility.