ClubCloud billing is based on the maximum number of simultaneous users you can have on video calls connected to each domain you have a licence for. We refer to this number as the number of Concurrent Users.
You might develop a site or platform that has hundreds of registered users, however only a portion of them are likely to be using video calls at any time. You will need to make a decision based on the business model you are developing as to what fraction of your usership will be online at any time.
For example a small business of just 50 people all of whom will be using video chat all day all together will need 50 concurrent slots.
But a much larger company of 5000 who only need to spend, on average 1% of the time on a video call will ONLY need 50 concurrent slots.
It depends on the number of distinct users and how much time each is likely to be in a video chat.