Probably less than 50 people that are willing to do more than one charter per year. The number used to be quite a bit higher, but rising prices from the railroads and an aging demographic have taken their toll.
The good news is, there are a number of younger folks running charters that are pretty much flying under the radar at the moment. These charters may not be up to the standard of Lerro and HTP, but they are learning and they are using museum lines where the prices are quite a bit lower. I probably won't be around when people are running charters with SD70MAC's as "historic locomotives" but I have no doubt that day will come and there will still be riders.
Michael Allen