If it helps, we have a 3 tier environment. Dev, QA, and Prod. QA and Prod mirror each other as much as we can, just with lower server specifications and less relays.
Dev - is a single server setup, with ~50 licenses. It also has a dedicated cloud plugin server and 2 relays (our needs). We test new versions, sites, features, etc here.
QA - Two servers with DSA, dedicated SQL boxes, a few relays, dedicated cloud plugin server. We use this to test our design, policies, new features we want to use from Dev, etc.
Prod - Two servers with DSA, dedicated SQL boxes, ~12 relays, dedicated cloud plugin server. Prod, though we have not yet moved into production as we are new to BF still.