I'm only parroting what I've heard before, and odn't have any 'inside' info. But, the max. capacity of Disneyland is based alot on the capacity of rides- such as next spring when Nemo is up and going they park will allow more guests in before it reaches maximum capacity.
The actual cut-off Disney does is a little below the fire marshal regulations. This is no doubt to be 1. on the safe side so they don't go over. and 2. Disney knows that once the park gets sooo crowded, guests start complaining and need to comped, that and they may loose a return visitor because it was too crowded.
The number varies and will never be concrete, but the most common numbers I've heard float around the extreme high 60ks to very low 70ks. Certainly not 80ks though.