In accordance with our guidelines, Wikipedia isn't the best source for information. At a quick glance, TMDb's data does not match up with what Wikipedia has, so I'm going to try to find official sources (ideally from Disney).

I know but these aren't episode These are extra

The official websites (Disney XD, CHRGD, Nerd Cops, Slugterra) were not very helpful. I followed the press releases and social media accounts (Facebook/YouTube).

• The show was originally announced as a 39 x 22 min series.
• The second season is composed of three films: "Ghoul from Beyond?" aired on March 30, 2014 , "Return of the Elementals" in September 2014, and "Slug Fu Showdown" in March 2015.
• The third season began with the TV airing of "Eastern Caverns" in January 2016.
• The fourth season is the TV release of "Into the Shadows" and premiered on Tuesday, October 4 2016 (x) (x)

we can take as source Amazon Prime Video? season 2 according to tmdb is 3 episodes, but amazon says it is 7. https://www.amazon.com/gp/video/detail/B094YYPKQZ/

No, because we have to follow the original release. To split them, I would need proof that they originally aired in multiple parts. The Amazon Prime episodes don't mean anything because they frequently use alternative seasons with episodes often in a different order and/or cut.