It’s hard to guess just what you’ll need. I know of a site with 3.5M monthly page views on a 8GB 4CPU VM. You could use that as a guildeline. It’s serving images itself, which you can’t (easily?) do with multiple instances; you’ll presumably have assets on S3 and CDNs for the site and the S3 bucket.
Whether a “request” and a “pageview” are the same is somewhat unlikely.
Whether you need scaling depends on your traffic patterns. If you needed scaling before, then you probably will with Discourse. If your traffic is fairly consistent and you don’t need scaling, then the easy solution is a single big VM, and probably RDS and maybe Elasticache. It also depends on what tools and methods your team likes to use, your budget, and so on.