If you are making a map/scenario you choose which of those civs you want to include. For example, when I made my Viking Scenario, I used the civs appropriate to the place and time... Normans, Germans, Vikings (4 different ones), Saxons/Anglo-Saxons (about 4 different ones), various Welsh/British, Irish, etc. The mod gives some 30 different civs to choose from, but you choose the ones specific to your scenario.
If you are playing the basic mod with generated maps, then there are 18 civs which are not optional in that case... the first post, I believe, cites them. However, for scenarios there are a wide range of possibilities.
That being said, there is some suspension of believe that is necessary in many cases. In my Viking Scenario, which is just based in Western Europe, many of the wonders are not appropriate... with BTS's mod component ability, I may be able to address this at some point. Similarly for the religions... I use Orthodoxy to represent Celtic Catholicism, for instance, and have put a little island in the NE corner of my map in the Atlantic that is a peak surrounded barb city into which I placed an Islamic/Jewish Holy City (the generation of Holy Cities is Hard-coded so I needed this work around to do it.)
Anyhow, if you are making a scenario/map, there are many ways the map/scenarion file can be manipulated to let you simulate almost any period during the middle ages. Your question leads me to believe that you may be unfamiliar with how scenarios and maps relate to the XML files so that the tools this mod provides for scenario creation can be best used.
If you give us information on the scenario you wish to create, perhaps we could advise you on how this might be done using the tools the mod provides.
I'd advise a seperate thread for this, though.