My GTD application Tracks has a little red badge at the top left of each page displaying how many uncompleted actions are in the list, and over the past few weeks, I've been keeping an eye on that number. Of course, everyone is likely to differ in the number of things that they have on their list; not only because people are differently busy, but also because everyone has a different level of granularity for the tasks they enter.
It's probably a failing on my part, but my granularity is quite chunky. This is partly because some of my tasks are difficult to break down into smaller pieces. For example, I'm putting together a poster for a scientific meeting at the moment. Now, I could divide it up further into chunks like 'write text' and 'insert figures', but that would be a bit artificial. I just have to get on and do it, but it's going to take more than the mythical half a day.
Possibly because of this chunkiness (or because I'm a total lightweight — who knows?), my magic red badge tends to vary between about 10 and 20 (this number doesn't include emails I have to deal with). I strongly suspect that the upper limit isn't down to my ninja-like efficiency in keeping the number of tasks low, but my reluctance — when under a lot of pressure — to add anything else to the list. The lower limit is more of a puzzle. Whatever I do, I can't seem to get it down below 10. Once or twice the number has tantalizingly flipped down to 8 or 9, but then I instantly get some other task to deal with, and back up it goes.
It's like some kind of physical GTD constant for me.