I think it is a hard limit imposed by the force selection screen. I tried to do a mod to overcome it in which you selected groups of different units and then with script you will create those units, so you could have 18 different groups.
If this is a hard limit imposed by default BA, it's definitely worth asking Pip to consider a fix for BA2...
Otherwise, it's really going to limit us for Pacific SP...
Yes, currently there is a hard limit of 18 unit types which are visible on the force selection screen. We can consider increasing this, but I think any kind of "scrolling" UI on the force selection is going to be pretty ugly.
Perhaps I will look at having it auto-scale if there are more than 18 types.