The question from the title is one of the most difficult ones to answer and it is quite possible that there is not a definitive answer to this question, even if we were to spend decades trying to find out all the various internet clubs that you can become a part of. The number grows even more when you start including the internet clubs that are not in English into the mix. And there are a lot of them. In fact, you might say that internet clubs have become one of the leading ways in which people with same interests and opinions share those interests and opinions and interact with each other. Maybe the best way to illustrate how many internet clubs there are out there is to try and think about all the various variations that you might have and all the various interests that might elicit the creation of a club.
For one, there are those general social internet clubs where the main point is to meet someone new to talk to or to start a relationship with. There are innumerable such websites in pretty much every language in the world, with millions of members from all the various parts of the globe looking to meet new people and meeting them. These internet clubs are huge and there is no chance that you have not seen or heard about at least one of them. They are a great way to meet new people as you can learn about them beforehand and as you can approach those people that seem interesting to you, which is something you just cannot do in person.
Then, there are more specific internet clubs and one of the variations are the dating websites or even adult dating websites or clubs where you go in order to meet someone you will find attractive and someone you would like to get romantically involved with. These are a huge business and they can be of invaluable help to people who are shy or do not have the time to date using the more conventional channels, so to say.
Of course, there are also clubs that cater to people with specific interests. For instance, you will find clubs where history buffs meet and discuss their views on certain historical facts and controversies. There are people who are interested in American football and who love exchanging their opinions with other people. There are also clubs where you will find people discussing Asian cinema or any other art phenomenon from history or from present day that is a more esoteric subject than TV shows that are running at the moment. In fact, you might say that every website that includes some sort of interaction between members (which is every website nowadays) is something of an internet club.
As you can see, it is absolutely impossible to establish how many internet clubs there are, but the number is definitely a huge one. One that would blow your brain. Who knows, there is probably an internet club where this is discussed.