'instant' means concurrent
You're right, I was about concurrent visitors (those hitting website once in a time period and which are handled by miniBB's Who's Online add-on).
I doubt that will happen for a very long time. :)
If you don't plan to invest a lot in promotion, and/or if your niche is not absolutely unique and hardly missing on today's online markets, then you will spend some time to reach some huge visitors amount. By that moment, you will surely collect more infos about your configuration and server load and then you could switch to a more powerful VPS or dedicated plan *if ever needed*.
Actually that's about the Philosophy of miniBB... you start from basic things, smaller things. Then if you grow, you move to more difficult things and add more features to your project, this also may affect the hosting plan.
In the ancient ages, we were keeping miniBB on a very basic shared hosting, but as soon the amount of visitors started to come up to 1,000 hosts per day, then we moved to another server. I guess on VPS the amount of hosts could be quite bigger (but still it has a border line which couldn't be crossed).