Newbie Here
Using Jamroom
You can have a paid subscription system.
Jamroom Hosting is all-inclusive, so you get the hosting and all the modules/skins made by the-jamroom-network including premium ones on as many domains as you care to create.
"Jamroom Hosting"
https://www.jamroom.net/hosting
You set it up from your profile, so for you thats here:
https://www.jamroom.net/scottbland/hosting/overview
Prices start from $39 / Month.
How many members isn't so easy to guess at, it depends on what your members are doing. If they are uploading a lot of video files, then your server will need to convert those video files so there is a format available for all devices as well as serving the website when users come to look at it.
If you're just a forum site all the server needs to do is deliver the site, so many more. When you start to notice that your server is slowing down, upgrade.
Jamroom is designed to be useful to developers, so sometimes it can feel daunting to users who've never built a website before. Its very customizable and expects that you will want to customize it for your needs.
If you click the free trial and create your server you can have a play around with it, or you can download the free version and install that on a server you already own to check out the admin and members sections. We dont have a demo of those up.