How many Seasons / Episodes are there really?

There is either 1 or 3 series, depending on your definition.

There is only 1, four-part serial called House of Cards.

Then, there are two follow-on serials, also of four episodes each, called To Play The King, and The Final Cut. These were made with several years between them (1990, 1993, and 1995), rather than being a recurrent annual series.

Also, they are three seperate serials, each telling a largely self-contained story, with a conclusion at the end of each, rather than a narrative cliffhanger.

Unlike an ongoing series the narrative doesn't follow directly from one serial to the next - but it's understandable that they can be regarded as Series 1 to 3, and for ease of reference (DVD Boxed sets, for example) these days they are often called The House of Cards Trilogy.

Anyway, that's why IMDB lists only 1 series, but you'll find people talk about more.