Mexican 2nd division
The Mexican 2nd division consists of 24 teams that are split into two groups of 12. Each team then plays 2 matches against each team in its group and one match with each team from other group. Thus, each team plays 34 matches per season.
However, to make it a tad more interesting, there are two stages of 17 matches: The Opening stage and Closing stage. The top 4 teams of each stage take part in championship play-offs after the end of each stage. The winners of the championship play-offs participate in a promotion play-off at the end of the season, from which the winner gets promoted.
As far as relegation goes, there is a table of all 24 teams sorted by average points per game coefficient for last three seasons. At the end of the season, the bottom team on this table gets relegated and 23rd team goes into relegation play-offs.
You need to have 5 under 23s in your first 11 at all times!
Source: Vladimir Kocjancic