Hi,
You need to configure multiple arfcns in yatebts. By default you can only have:
* Call capacity: 7 concurrent full rate call
YateBTS doesn't support half rate, so you can't increase call capacity this way. From yate's point of view 20 ivrs aren't a big number, but running multiple arfcns takes more resources and decreases coverage. In the end you'll need to experiment. Let us know how it goes.
Good luck