Dear Aquatic,
I'm working in similar environment, only 24 carrier and over than 1 milion subscriber with high demand trafic !! . since you have only 20 site so I suggest to use 10 BCCH and 15 TCH this should work but u need to be carful with the BCCH planning, use AFP tool then check it manually.
I could help u in the planning if u want just PM me.
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