Once per entering close range, or once per turn inside of close range?
I have seen this played by GM’s in a number of fashions. I apply this once per scene when entering close range with the Ghoul and I increase the difficulty for every 4 Ghouls present. But I have played games where GM’s apply this test just once per scene. I have not played as once per turn but I could see this as a reasonable option.
I believe it is supposed to be once per scene. The ability functions much the same way that so many of the other enemies do. I do like the idea of increasing the difficulty based on the presence of more ghouls but that’s not official. Personally, I say feel out the encounter and decide what you want from them. If you want to encourage your players to do something to negate it, have it happen once per round or once per time a PC gets close to it. If you just want to have it be a possible inconvenience, then have it happen once per scene.