Hello Mark, you say 10k, a million, but what would be interesting to know is what server capacity?

Server capacity is not really governed by member numbers but more by member activity. How active are the members and what sort of content are they posting? A video site is obviously more server intensive that a discussion site like UNA.IO. As a site scales, remote storage is always an option to maintain site performance to keep down costs.

I'm no expert in this area, but I'd say that any site with much more that 20,000 members is probably going to need some professional consultation from a skilled technician such as Alex T.
Baloo have you seen this article?