Well they did know there was a set number, that’s how it was designed, it’s just that they didn’t ever think to check for more than their maximum number because they didn’t think that there was any reason to until they added another animal to the park, in which case they’d update the “maximum expected animals” number and everything would be okay. If I recall correctly, there was a system to alert them if the tracker reported for a number lower than expected, but never higher than expected, because they didn’t think it was possible. That’s why I wish they explained it more in the script here, because the book makes their hubris more obvious.

It’s one of the many oversights that led to the downfall of the park, as we’ll see.