It's a common metaphor to say “I'm juggling a lot of things right now.” And everyone knows there's a limit before you start “dropping” items from your consciousness.
The records for juggling multiple physical items only go up to about a dozen. And those records only last for seconds at a time. Maybe there's a correlation that nature is enforcing with juggling ideas in our minds. Maybe even the greatest mind is limited to 12 items, and even then only for seconds.
Three physical balls can be juggled all day by any of thousands of moderately accomplished jugglers. I think we'd all agree that keeping 3 items in your head all day is just as doable. Seven balls, on the other hand, can only be juggled for 10 minutes by the very best jugglers. So the next time you have 7 items to work on for the whole day, consider the possibility that isn't how your mind was designed to work.