I know you should get backlinks from as many urls as poss and that multiple backlinks from a single source start getting devalued. What is the optimum number of articles to send to one GOOD directory - eg one with a high pr that has good indexing? Would you say 2-3 or can you still get serps value at 5, 10, 15 etc?
Submitting to the same directory lets say for example Ezinearticles that has a PR6 would not be a bad thing necessarily. As long as you're not spamming them or blasting hundreds of articles at once there's no reason why your backlinks should get devalued from an authority site like them. But you should definitely consider submitting to different directories so that you get more leverage out of your articles. Look into using a service like Unique Articles Wizard to make the whole process more efficient and effective.
No I'm wondering whether it's worth having 5-20 articles in a quality directory like ezines or whether after a couple of articles you get no further benefit in the serps (I'm not interested in traffic).