On Page 40 of the Unofficial guide, you;ll see the annual averages for the MK as far as daily attendance.
Elsewhere in the book, they say how much the other parks do in comparision to the MK also at least on an annual basis. So the kind of info that the original poster requested is available.
Now the averages can go up or down based on new rides, early or late entry parks, the weather that day and the success of promotions offered. But I do thinking knowing past attendance can be useful in planning too.