I’ve heard 1,000 a day, and NaNoWriMo wants 1,667 or so a day (to be fair, if you’re following NaNoWriMo’s standard, you are trying to write a 50,000 word novel in a month), but I say as many as you can reasonably manage. I personally only used to manage about 600 words a day, because of my extremely freeform style and all the plot details being jumbled in my head. Some mental housekeeping has boosted that to about 1,400 a day these days.
Of course, if I had to give a number, I would say 500 a day is a good goal, as it keeps things moving at a slick pace without being too much of a strain to come up with new ideas. Also, if you’re writing a 60,000 word novel you’ll get it done in four months.