I'd hope for 10 in the top 10 league, then up to 20 in the second league before a third league would be needed. Top 2 get automatic promotion, the guy in third would have to have more points than the guy 3rd from bottom in the top league to get promotion, for a maximum of 3 promotions and 3 relegations. If someone drops out after a another person gets promoted.
It would give people who find themselves in the middle of the pack something to play for. Relegation battles are always fun too. It would need to have a season to determine placing in which league.