How sure are you will see this kind of activity from day 1? Where are you getting the figures from? Are you migrating an existing forum?
Most hosts offer very scalable solutions. Start with the best you can budget for with a host that has scalability both ways. One thing is that discourse values fast CPU over a group of slower ones.
With images it might be best to consider a “cloud” storage/delivery service like Amazon S3 which is supported by Discourse. This will help with hosting costs, efficiency and visitor experience by offloading image duties to Amazon’s infrastructure.
Multisite could be challenging from a technical and community perspective. Personally I would engage a developer/dev ops person of you choose to go that route. Even if you are capable of the dev ops yourself, it’s probably not the best use of your time.
Otherwise you could setup categories for cities as you roll them out and set the subdomain to redirect to the URL of that category.