5 was an arbitrary number. I have daily items on my calendar that have to be done every week, every day. After that I have ad-hoc single actions that pop up on a daily basis that need to be added. Then I also have customer orders that dictate my time as well. After all those, I find time to work on my projects in whatever context.
With all of these I make sure I have enough NAs to keep all my contexts full. My ultimate goal is to NOT have to scroll down on my pda to see the NAs in any given context. If I have to scroll, I know I need to start getting things done because it takes too long to have to scroll when switching contexts. With this method, if it's in the context, then I do it!
I also don't stick just to a Weekly Review. I review NAs and Projects almost on a daily basis to keep everything moving. My Weekly Review is a little more involved on a Friday evening. If I finish a Project on a Tuesday evening, however, I have no problem checking in S/M to see what I want to move up to active right away.
I simply know how I work, and I need to keep a small amount active, and pick which Projects I want to "activate" during my review. There simply isn't enough time to keep many projects going. I like to complete, remove, and start a new one. It keeps me moving without getting numb to seeing too many NAs over and over.