21 BTC... and be one in a million. You can continue on from there, but for most of us, being one in a million in regards to BTC is a fantastic benchmark.
Depending on the individual income, i would say starting with 1BTC.
And then increase it until 10 BTC, then until 21 BTC as centauribit said.
The higher the amount that we hold the higher the risk is.
21 is too much for many people. Only hold as much as you can afford to lose. I don't think many people have $4,800 just lying around as extra money.
I still think that 1 is the magic number. It's very affordable right now to many people around the globe and when bitcoin takes off in 2, 5, 10 years one bitcoin could be worth a fortune.