Hello Gavin,
The most important requirement for deploying ACI in Application Centric Mode is have the detailed Application Dependency Matrix (ADM).
Based on the ADM, servers with the same instance of the Application are grouped together within a particular EPG. And, the communication between the EPG to EPG will be controlled via Contracts.
One BD per subnet is the ideal start for any form of deployment.
Regards,
Harshal
