Right now I am running 6 without any problems. It's a challenge for me to keep the count low. I have a lot of 62.5 micron fiber so gigabit won't go that far on the old fiber. Upgrading all of my fiber would cost big bucks so I am stringing a couple of gigabit switches in a row and using 100FX on some backup links since it goes farther on 62.5 multimode.
