gui design - How to determine how many widgets is too many widgets?

Following on from my last question, we're working on a UX design for an application. Once logged in, the user is confronted with a dashboard, something in a similar vein to this; just more complex and prettier - simply it's a dashboard of widgets that display all different sorts of information from all different areas of the application (picture this as a top level, it's a really complex job).

How would I go about determining what would be the optimum number of widgets to display to my users?

Enough to fill the top viewport? So 9 or 10, maybe more?

Are there other cogsci or usability factors to consider?

I know this might be an opinion based thing as well, but I would really appreciate any sources and reading material that is related to this too as it allows me to pitch better to the client when they ask my reasoning behind why I need to limit the amount of widgets they are allowed to display.

I'm doing my research too, just wondered if anyone here has had this problem before.

Answer

User testing is the only way to answer this question. Specifically, I would make some realistic page examples and poll your user-base to see what they like.

EDIT: As a general rule, though, I would agree that filling the viewport above the fold (if there is one) is probably a good idea.

I'm a developer and I find the default Google Analytics dashboard positively naked; I showed this to my father, and he found it impossibly busy.

(image taken from my alma mater):

There is an easy way around the problem, though: allow the user to customize what they see. That way it will at least always approach the range they're most comfortable with. You just need to design your widgets sensibly, balancing how much information you want to put on a single widget given its role: the more atomic the widgets, the more widgets you will need.

EDIT 2: Are you planning on deploying to mobile platforms? If so, the design of your mobile layout may help to inform the design of your desktop layout. Presumably the mobile dashboard, if any, would be more restrictive due to lack of available screen real estate, but if you can "get away with" X minimum widgets, perhaps a similar number on your desktop would also be appropriate.