In that case, you'd have to ask them personally. As for the number of people who use it, there's likely no approximation.
You see, it all depends on what people use PokeCheck / PokeSav / PokeGen for. I, for instace, used it for my SoulSilver file just so that I could fill up the Pokedex with PKMN that I can't find/evolve in the game itself. Others use such programs to make invincible, ridiculous parties, and other simply use it to find out what their Hidden Trainer ID is. There are a lot of uses for these programs, so there's no real way of telling how many people use it.