Perhaps the most violated eRoom best practice we've seen is a glut of documents and items at the top level. Nothing creates information overload and confusion more than sheer quantity of items in an eRoom. It's not like a website home page where you can at least control size, placement, and image type of content. Nope. You are limited to 32px icons spaced evenly (in large icon view).
When a user enters an eRoom for the first time, they should be treated with a pleasant experience. I joke with my wife when I tell her if we want get the house cleaned, invite some friends over. For some reason we cannot bear the thought of our guests gazing upon a myriad of legos, sports gear, shoes, and books scattered about the living room. Be equally nice to your colleagues and don't let their first thought upon entering your eRoom be, "Oh my, who spilled their network folder here?"
So what's the number? We suggest no more than 12 items at the top level of an eRoom. Fewer is better, but after 12, the user has to process more than necessary.
Are there exceptions? Yes, absolutely. For example, we created an eRoom for a client used by their Trade Show department in which the 40+ trade shows they attended each year were top level items. In that case, all items were of the same kind and ordered, so it made for a quick index.