i started with 500 backlinks a week and just kept at it
still haven't been sandboxed and i have uped it to 1k a week and my rankings have jumped from the 9 million to 478k in alexa and from PR-N/A to PR1 in 2 months
The number is not as important as choosing an amount you can sustain long term. What you do not want to do is build a bunch for a week or two and then stop. Choose a number you can continue to do for a few months and then go for it. Ten links a day or a thousand, as long as you can keep doing it then you will be OK.
That's the part I suck at. I get a good start and then wander off to promote something else.
Planning, automation, and outsourcing are your friends.