How many episodes did the second anime season of Nanatsu no Taizai have?
As of 2019, officially in Japan, there are 4 series, but only 3 seasons:


Nanatsu no Taizai (2014): 1st season, 24 episodes
Nanatsu no Taizai: Seisen no Shirushi (2016): TV Special, 4 episodes, in which the last episode revealed the announcement for the 2nd season (see also Why is Nanatsu no Taizai : Seisen no Shirushi only 4 episodes?)
Nanatsu no Taizai: Imashime no Fukkatsu (2018): 2nd season, 24 episodes
Nanatsu no Taizai: Kamigami no Gekirin (2019): 3rd season, ? episodes


However, localization (and sometimes fans' opinion) make season count inconsistent or confusing, as Netflix considers each series as its own session:


The Seven Deadly Sins (2014): Season 1
The Seven Deadly Sins: Signs of Holy War (2016): Season 2
The Seven Deadly Sins: Revival of The Commandments (2018): Season 3
The Seven Deadly Sins: Wrath of the Gods (2019): Season 4