How many professional football leagues are there in the U.K.?
English football league system
Premier League (level 1, 20 teams):  The bottom three teams are relegated.
English Football League Championship (level 2, 24 teams): Top two automatically promoted; next four compete in the playoffs, with the winner gaining the third promotion spot. The bottom three are relegated.
English Football League One (level 3, 24 teams): Top two are automatically promoted; next four compete in playoffs, with the winner gaining the third promotion spot. The bottom four are relegated.
English Football League Two (level 4, 24 teams): Top three teams are automatically promoted; next four compete in playoffs, with the winner gaining the fourth promotion spot. The bottom two are relegated.
National League (level 5, 24 teams): The champions are promoted; next six compete in playoffs, with the winner gaining the second promotion spot. The bottom four are relegated to either North or South division as appropriate.
National League North and National League South (level 6, 22 teams each, running in parallel): The champions in each division are automatically promoted; next six teams in each division compete in playoffs, with the playoff winner in each division getting the second promotion spot. The bottom three teams in each division relegated to either Northern Premier League, Southern League or Isthmian League as appropriate. If, after promotion and relegation, the number of teams in the North and South divisions is not equal, one or more teams are transferred between the two divisions to even them up again.
Northern Premier League Premier Division, Southern Football League Central Division, Southern Football League South Division and Isthmian League Premier Division (level 7, 22 teams each, running in parallel): The champions in each division are automatically promoted; next four teams in each division compete in playoffs, with the four playoff winners paired off geographically for a further match to determine two more promoted teams. The bottom three teams in each division relegated to a level 8 division as appropriate. If, after promotion and relegation, the number of teams in the divisions is not equal, one or more teams are transferred between the four divisions to even them up again.
Northern Premier League Division One North, Northern Premier League Division One South, Southern Football League Division One East, Southern Football League Division One West, Isthmian League Division One North, Isthmian League Division One South Central and Isthmian League Division One South East (level 8, running in parallel, 20 teams in each division): The champions in each division are automatically promoted; next four teams in each division compete in playoffs, with the five best play-off winners by points per game being promoted. The bottom two teams in each division are relegated to a level 9 division as appropriate. If, after promotion and relegation, the number of teams in the divisions is not equal, one or more teams are transferred between the divisions to even them up again.