I'm no expert on consul, but I think it's architected such that you'll run a few servers (say 3 or 5) and the rest will be agents.
All machines using consul will (generally) run the agent locally.
All machines using consul will (generally) run the agent locally.
In general you are going to set up an N+M topology of n servers and n+m agents -- but for a network of 100k nodes does consul scale to 100k agents? Plus what goes unstated is how those 100k nodes are laid out in DCs and racks.
Anyway, this isn't to slag on consul. I think consul is like chocolate and peanut butter.
I use it in my mesos cluster and just am genuinely curious how large it can scale!