Ideal number of green clown gobies?

From my experience 1 clown goby per tank. I have tried, as well as my neighbor to have more than 1, either all yellows, all green, or a mix of the two, and after a week every single time there is only 1 left in the tank.. the rest are killed, carpet surf, or jump into the overflow and end up in the sump. I don't know why they can't get along like they seem to at the LFS. For a while I really liked those fish and wanted a few.. but after 4 attempts myself at having more than one, and my neighbor trying 5 different times, all with the exact same results, we have given into the fact you can only have one unless they are a mated pair.. which we have never seen for sale at a LFS.