I currently have 27 icons on my 17-inch G4 iMac's bottom dock. As you ad more it just makes all of them smaller to fit. I've yet to find a limit. On the other hand I only keep stuff in the Dock that I use frequently to semi-frequently. A lot of people keep an icon to System Preferences in the Dock, since I don't use it that often and since I can easily get to it from the Apple menu, I don't keep it in the Dock.