A user has provided a save with (supposedly) 14441 guests from RCTC. Sure enough, trying to load it up in OpenRCT2 results in the error message "Tried getting sprite 14293".

Download is here: https://www.dropbox.com/s/w09jqcnxml7i5cz/Forest%20Frontierstest_big.sv6?dl=0

I don't know how they do it.