Counting people entering the grounds won't be any use unless they count the number of people leaving and then work out how many people are still on the premises. With events running for several hours people are coming and going all the time. And there are 3 (or 4?) entrances to monitor.

Plus some people would be visiting the museum or another part of the gardens, not the event itself, so would be at risk of being prevented entry to the museum/gardens because the threshold number had been reached.