Sometimes you won't find enough people so you need to start talking it up the minute you arrive and you need to be flexible which night. Talk to Franz when you get there. Last time I was there it was $50 USD per diver.
My hubby and I went on a night dive at CSS last fall. We just asked everyone on the morning dives each day if they would be interested in doing the night dive... after a couple days, we had another couple who wanted to as well. Just let the guys at the dive shop know you're interested, and drop some hints to your fellow divers on the boat. You'll surely come up with another couple wanting to do the night dive.