If Simic Ascendancy doesn't have twenty or more growth counters on it as your upkeep begins, its last ability won't trigger. You can't take any actions during your turn before your upkeep begins.

If the last ability does trigger, but Simic Ascendancy leaves the battlefield, use the number of counters it had on it immediately before it left the battlefield to determine whether you win the game.

An ability that triggers when counters are put on a permanent will trigger if that permanent somehow enters the battlefield with those counters.

If the last ability does trigger, but counters are removed from Simic Ascendancy so it has fewer than twenty remaining on it, you won't win the game.