There are a bunch of haunted forest modes---haunted scoring for shots, targets, etc. (They're listed on the sign.) One is lit at the VUK after seven pop bumper hits (by default), spelling "HAUNTED" in purple on the arrow that points at the pops.

After you get all the modes on the sign done, the next one is haunted multiball.

Think Otto modes and Springfield Mystery Spot from Simpsons PP.

I mean, they're all frenzies for something, so it's not a huge deal to know which one is next. I don't have a WOz handy, so I can't tell you how many there are, but yes, they're only listed on the monitor. (I assume so they could change them after playfield was finalised.)

As far as I understand you won't get the same one again until you get all of them.

Also learned recently (thanks, pinside!) that you can disable the pops during play when a Haunted mode is lit, so if timed correctly, to increase the chance that the ball will fall through the pops to the left VUK.