How many extra-curricular activities would you put on a CV?

I wouldn't put going to the gym on a CV, it will make you sound like a meathead, unless you're applying for sport or something like that. Debating is a good thing to put on. Musical instruments, if you can play them, are good to put down, and competitive sport, such as if you have won any tournaments, or big competitions, or even if you just play for a team or in your spare time. Sport can show that you have physical skill, and team working ability, but gym doesn't show any of these things.

There is no minimum. Of course, a greater number of extra-curricular activities displays an active interest in more than just school work, and also shows that you can achieve good grades without spending all your time working.

To be honest, not very many unis care hugely about extra-curriculars. Activities in areas relating to the subject you're applying for can be quite useful, as they show that you really are interested in it, but even then they're often only used to decide between two otherwise equally qualified candidates.

Chances are your grades will be much more important. However, I'd still recommend joining some other clubs to show a bit more breadth. Maybe a sport? And some community volunteering or something similar?

working in a team, logical thinking, confidence, public speaking, ability to construct an argument, persuasive thinking etc
For the gym you could talk about how it requires a large amount of self-motivation to meet set targets, perseverance, ability to manage time effectively

I usually put two. They are quite important as they differentiate between you and the hundreds of other graduates with a good degree.

What matters most is not what you do but how you talk about it, there's no reason why you can't put gym-ing down especially as your other activity involves working in a team, logical thinking, confidence, public speaking, ability to construct an argument, persuasive thinking etc
For the gym you could talk about how it requires a large amount of self-motivation to meet set targets, perseverance, ability to manage time effectively to allow you to go to the gym 3 times a week or whatever, plus anything that you've stuck to for several years shows that you have staying power and that you are able to use your own initiative, so if you've been regularly going to the gym for x number of years it shows a large amount of dedication.

Rather than make things up that you have no interest in, stick with what you have got and think about how you can talk about them to meet the competencies that a lot of graduate jobs require.

Would I have to mention what competencies are met by doing these EC's, in my CV? Or would the employer interpret the competencies already just by looking at the interests in my CV?

Is it really necessary to put hobbies and interests on your CV (I'm talking about graduate jobs here, where you already have a strong CV notwithstanding the hobbies section)?

I don't have any remotely interesting hobbies. Is it worse to put down a few boring, generic hobbies - or none at all?

It is worse to put down the classic CV padders of walking, travelling, going out with friends and the cinema. It is also pointless to put in Interests that are just single words like

Interests: Fitness, travelling, music, potholing

However, you have to have a stronger reason not to put an Interests section in the to put one in. Interests, especially if you have limited work experience, is a good section in which to continue to demonstrate relevant skills.

I'd say only leave it out if you a) have an otherwise strong CV and b) know the employer doesn't care about work life balance and doesn't care about you as a rounded person. There won't be very many of those.