On Earth, it's virtually impossible to enumerate all the different cultures and ethnicities that exist, but if you hypothetically categorized every people as occurring with a language, that's roughly 7000 different cultures/peoples on Earth. (If you were to not do that, there are seven widely accepted cultural regions on Earth.)
With this knowledge, on Planet X, a Mars-sized body with an evolutionary timeline for its intelligent life much like our timeline (with tens of billions of years of adaptation), there are two warring peoples with their own two distinct cultures. Just two.
People Y live in cold, barren ice deserts that expand from the planet's poles. People Z live in lush, overgrown marshy forest that is wrapped like a belt around the planet's equator. Ocean separates the two regions from each other and split parts of the regions up.
Given the fact that there are only two distinct climates on the planet, would the development of only two cultures be plausible? Should I include sub-cultures inside the two main ones to account for separation distance and island isolation?