Just bought and downloaded this game. How do i play the normal version. Only option i get is the extreme version with 20 missions?

You should have two .exe files in your installation folder, one for each version (Stronghold Crusader.exe and Stronghold_Crusader_Extreme.EXE). You can start the regular version of the game by running the Stronghold Crusader.exe. To make this easier, you can create one shortcut to the appropriate .exe file, or change the path in the one that already exists.

If you are using Galaxy, this thread explains how to select the regular version: https://www.gog.com/forum/stronghold_series/galaxy_choose_between_crusader_and_crusader_extreme

Actually, it's even simpler than that.

Just choose Stronghold Crusader HD in Galaxy, click on More, then Other, then Stronghold Crusader HD. Why they did this is beyond me, but it's there.