Learning and using idioms like native speakers is something that you do when you are trying to master a language. It isn't necessary to learn big lists of idioms to communicate and speak fluently in English. In fact it would be pointless, and your time would be better spent doing other things because, as has been mentioned if you don't know how to use idioms properly it will make what you want to say unclear. And that is very the last thing you want to happen. The best way to learn them is you hear someone use an idiom or phrase you don't know, then take note of it and also how the person used it. The context will be key.

No, it isn't important. Use of idioms has nothing to do with fluency. It is much better to concentrate on ordinary, straightforward language, used correctly and appropriately. Nobody ever needs to use idioms. In fact, inaccurate attempts to use idioms can actually damage fluency, as people will not understand you if you get an idiom a little bit wrong or use it in the wrong context.

It's not something that you can count!

Yes, you should learn phrases, but there is no need to learn idioms.

Well, that's how my mom used it--and she was most definitely US, not British--and it is also the title of a (very good) play by the very U.S. playwright, Thornton Wilder, "The Skin of Our Teeth," so it certainly is or has been used in the U.S. And yet, I've never heard it used myself, except by my mom and in that play. The point is, it's not CERTAINLY important for a foreign speaker to USE it, and I don't think there's any reason to learn it--90% of the time you could guess it from context.