chawki $5 gives you 1vcpu + 1gb memory, so that's what discuss.flarum.org has been running on for a long time. They recently changed plans, the $5 tier previously had only 512MB which would be harder to work with primarily due to the need for composer to require a cache.
Your resource requirement really doesn't relate to the number of users, discussions and posts up until a million if those users hardly ever visit the site. The better metric to understand would be concurrent users, meaning actual people on your site at the same time. A regular vps of 1gb/1vcpu should be able to cope with at least 5 to 10 concurrent users.
But this also depends on the features you rely on in your Flarum instance. For instance if you use follow tags and the queue functionality (number of workers), your resource need increases based on the tag subscriptions as well. So even though I say "should be able to handle", it all depends on your specific use case, your ISP, your Flarum configuration and much more. I guess this is a disclaimer of sorts.
I am willing to offer advice about resource need to anyone who asks for it, but in the end it's better to start low and scale up whenever you need. Most (cloud) hosting providers allow upgrading your node quite easily, even if this causes a short downtime.