I picked 4 because that's how many free GRE score reports I got. Honestly, I didn't give it as much thought as I probably should have.
I don't recommend you do that : )
If I knew then what I knew now, I'd apply to a few more schools -- ones that take in more OOS people. I am on a waitlist, and I have an interview but it would've been nice to have a few more schools to hear from and keep my options open. I would have not applied to CSU, since they get like 1700 applicants for maybe ~20 spots, with $40,000+/yr OOS tuition.
I applied to OKstate, WSU, Oregon, CSU -- washington resident
If I could travel back in time... I would have chosen
OK state, Oregon State, Alabama, Miss, WSU, Western
Just keep in mind where you want to live, the tuition, and if you think you'd be a competitive applicant (somewhere is a list of the schools and the GPA/GRE averages for admitted students)