For example in gen 1, there was no sp. def and sp. atk. I don't know when the six stats appeared. (HP, attack, sp. atk, defense, sp. def, speed)
So my question is in gen 2 games, are there already six stats or only five?
No. In gen 2, there were 6. They are: HP, Attack, Defense, Speed, Special Attack and Special Defense.
In gen 1, there were 5. They were HP, Attack, Defense, special and speed