I think it depends what we are talking about.
For a 'daily' list 5-7 seems about right (depending on what they are too, some people prefer less or more), you can put more stuff on weekly or monthly or 'timeless' list/s...
Daily/weekly/monthly lists are not strictly GTD, for some people (including me) 300+ next actions can be overwhelming so I prefer less and a more structured approach somewhat..
It also depends upon your system - do you have paper or computer based system, can you just look at one or a few next actions at a time, or do you have to look at all of them etc. If you can just view a few at a time, it doesn't matter if you have 300+ stored in there somewhere... If you'd need to look at 300+ each time you look at a list, that can be overwhelming.. (at least for me)
So the idea is to have so many that it 'works' - not too many (that would be overwhelming) and not too few (so you wouldn't get important things done)..
Actual numbers depend on individual circumstances...