Not that I'm aware of, but I've never done any tests.
Are there any in particular you're looking at? And by ‘plug-ins’, do you mean extensions of all types, including commands and command panels?
There are only two downsides that I can think of: 1) If you install a lot of plug-ins at once, and begin experiencing a new Fireworks bug, you may find yourself wondering which of the many plug-ins may have caused it, and 2) The more commands you install, the harder it can sometimes be to find the command you're looking for. It's like toolbox with too many tools (all of them highly specialized). These both seem like minor concerns, though.
My Commands folder contains 4,206 files in 908 folders, so I haven't hit a limit yet.
Command Panels are a slightly different story. There's no limit to how many can be installed, but each one you have open does consume resources, as each one's basically a Flash movie. And since they can respond to events, like a new selection, each open movie has to process that event. So having too many panels open may slow things down.
As for finding that one command you're looking for, the QuickFire and Commands extensions can help: