Hi,
The script is designed to hold an infinite amount of users. What comes into play is the power of the server(s) the script is hosted on. Yes, you can have 780,000 members in the database and that is just data sitting there. When members are online and actively browsing the site and using its functions, that is where your power consumption comes in.
So say you have a top of the line server with 16 processor cores and 8 GB of memory.... That will be good for many users. But what if even that gets maxed out? Well then you can invest into clustered servers that will share the load over the span of how ever many servers you rent. You would like to have the fatest network connectivty available as well to go along with this.
I have never read a study on this before. These are just my random thoughts, but should give you an idea of the steps you would need to take to keep up with the demands of a world of users. Start with one server, top of the line. That one gets maxed out, add a second server that is clustered to the first. That gets maxed out...... keep adding servers. There ya go :D
May be a better way to do this so this post is open for input. However, do let us know when you get to the point of needing serveral servers. I would like to see that Dolphin script in action....