Attendance numbers are a tightly held secret but when I was a CM the "ballpark" maximum capacity that the parks would reach before closing was:
Magic Kingdom: 70,000 guests
EPCOT Center: 70,000 guests
Disney-MGM Studios: 30,000 guests
(I left in 1993 so keep in mind that the Studios have expanded since then and Animal Kingdom wasn't open yet).
Trust me - you do NOT want to be in the parks when they are at capacity! It is miserably crowded everywhere.