Indeed, there’s no hard limit to number of users or amount of activity - it’s just that the forum will be slower to load and refresh as it starts to struggle with memory.
And on the topic of memory, you should expect your RAM to be used. You will have swap, and that too will be used, and that’s one thing which can become a hard limit. Keeping an eye on free swap and on paging activity will let you know how that’s going. (Use top or free and/or vmstat for this.)
And yes, disk space is another hard limit which might be limiting, and probably the first one you’ll hit - the biggest variable here is how many uploads your users make and how big they are. You’ll either need an instance with more disk space or you’ll need to split the upload storage to a cloud storage provider. There’s information on that, but I haven’t yet done it myself.
You do need more free space to update the software than you do to run the forum, so every few weeks when an update comes around you might need to see how you are doing. Downloading and then deleting your backups can help.
I wrote more here: