You can have as many as you want included in your map and up to 16 on the map. Only the first 2 will be used however (with the rest messing up in various ways). For some reason the game supports up to 16 cliff types but will only load 2.
You can make up to 4 cliff types by getting 2 varieties of the cliff geometry per cliff type. Both natural and artificial cliff geometry can be used for the same cliff type (not sure what controls it, I thin the cliff variety field has some underlying structure to it).