Clearly you don't pick a single digit or double digit number. Those are all already taken by someone who thinks they're clever enough to be the only one to do it. Also as clear is to not pick 1 million or a sextuple digit number. Then you're left with triple digit to quintuple digit numbers, and that's where it gets a bit chancier. You can do some rough calculations based on total playerbase and percentage of it that you think will play in order to estimation distribution of number choices - I think it's easy to see that the number choices from 100-99999 will be weighted towards the lower numbers, so as you want to avoid repeats, you try and pick a number that's reasonably high, but not pushing the extremes (say not above 50000 but not below 8000 - different bounds can be made based on different assumptions of distribution, which depends on the data you can get).
It does require luck, bob, but there's more strategy than you think.