as many as you are committed to...
if you've listed all your projects and have defined NA's for all of them, then you should be fine. For one person that might be only 5 NA's, for the other one 55, for yet another 100... I usually turn around 100, and those are only work-related, not private...
But then again, I'm a consultant, with a multitude of smaller projects going on. If I would be the type of consultant with 5 big projects to work on, I might have only 5 NA's.
Don't forget after all: NA-list is not your todo-list for the day/week/month/... it's where you left off working on a project and where you should pick it up again. So I might have 5 projects, with 5 NA's and work on only 1 project today, and tomorrow thus I'll have 5 NA's from which 4 are the same as on my list I have today.
I know there are people who don't like to see 100 NA's on a list, and will filter the list to whatever actions they want to execute this week. I don't like that, I like to have all of them visible, so I can pick whatever action I want to perform, even if I should not absolutely do it now (for example print a text I'm supposed to read in 3 or 4 months, if I'm in "printing modus", then I might as well print it and allready put it in the read/review box).
Myriam