You can design it the way you want. The way I built mine is as follows:
- 1 cluster per DC (US-EU-ASIA...) = 1 server group in Enhance
- Within each DC: 1 BM for WP hosting + 1 BACKUP server. It's important to keep them both in the same server group.
- CP is placed in it's own server group in Enhance.
As for the DNS, you can use any server as the NS. I enabled the NS role on the CP and each application server (BM hosting WP). That's all. Then I set my glue records on the domain accordingly.
You can have as many NS as you have servers.
Does that make sense and answers a bit your question?