How many controls in a tab page?

I was trying to refresh my skills on tab controls and tab pages reading Erik Brown's very good Windows Forms Programming with C# when I found this pearl:

"As a rule of thumb, make sure the controls in each tab page are all related, and try to limit yourself to not more than seven controls per page. The number seven here is not arbitrary, as user interface research has shown that this is a reasonable maximum number of items to present to a user at once."

The bolding is mine. Unfortunately, he doesn't point out to the specific source but I suspect it is Ben Schneiderman's Designing the User Interface (amazingly, it seems like it's not available in Amazon).