Thanks to Penguino for posting this in General Discussion I have copied and pasted as well as pinned this so it can be easily accessed
Before the scenario section in the zoo.ini, add these lines:
[ai]
maxGuests=x
where x is the number you want to set as the maximum.
You can change the number of allowable visitors to your zoo with the above formula by opening the zoo.ini file in notepad. don't forget to make a backup copy of the zoo.ini file first and save it somewhere outside the ZT game folders so you can replace it when you want to change it back to default.