I have 8 now and so far so good...but have lost about 2-3 over last 6 mos from since introduction. Want to add more but I too have heard and seen the tales of missing chromises. LOL I added 6 then 4 more and as I said ended up with 2. They mostly hang out together especially at feeding and roam the tank in groups of 2 or 3 at a time until they form up back to the main 8 chromis formation to cruise the tank.
After I could not find the first two I stopped looking. If they were not on the bootom I was not going to worry myself about.
The first two that came up missing it was like a Houdini act and I said screw it. I am not looking anymore. Since this post I have moved on from the decision of getting more Chromis and I am now thinking maybe 3 or 4 anthias lol. I think they are safer.
I wouldn't get chromis right now. More and more of them seem to be shipping with disease and as shown in this thread they are a real crapshoot in terms of ability to keep them in groups and they don't seem to do very well as single fish either.