Get 10-15 and you might be left with 4-6. I think these must be cyanide caught often, they always come up missing! My buddy has a 400gal and he put in 100 (ok 98), he had the LFS special order them for him. In 3-5 days he only had 6 left and 2 weeks down to 4. All other fish and coral was unaffected and water was in perfect parameters. It turned me off to chromis for good!
What if 10 people had each bought 10 fish, I think you would have 10 people thinking it was thier tank, when the fish were swimming corps to begin with.
Others have had the same chromis (or 7) for years with only happy schooling to report.