Why am I so shy around big crowds, but when I'm with one friend than I'm fine?

You're simply ambiverted. I'm ambiverted too. I'm the craziest one between my friends, but when I have to talk in front of people, or I just have to be around them, I'm an anxious mess.

It is possible that you are experiencing some social anxiety, or it just takes you a little longer to become comfortable around people. It is fairly common for people to be bothered by big crowds, as long as it doesn't interfere so much to the point where you feel you are unable to do things that you want or need to do.

Well - Because you know the friend well and not big crowds who are strangers to you. Think about when you remember meeting your friend for the first time and shyness came out!

Anxiety is something that happens when your brain thinks you're in danger. For me, it is something that happens all of the time but it's less severe when I'm around people I know and love. My brain knows I'm safe with them, so my anxiety doesn't get triggered all that much. So what you're experiencing is normal. Big crowds are probably more likely to make you anxious, even the mere thought of big masses of people scare me. Don't take my word so seriously though, I am definitely not an expert. I'd recommend you to go see a professional if you feel like you're dealing with anxiety, they can help you more than I can, and it really helps.

maybe it's a kind of agoraphobia ... but i guess it's normal to feel impressed when there is so much people around

When you are around big crowds, you often tend to be more shy because you feel as if you are open for full judgement. But when you're with your friends, you don't care because you know you can be yourself.

When you are around you friends, you know they accept you as who you are, when you are around a big crowd with people you don't know, you are scared you'll do something that they may not like and think different of you.

You do have to make yourself comfortable like you do with your friend because you don't think what your friend will think of you, in the same way you shouldn't think of people around you what they think of you.

this sounds like a symptom of social anxiety. Social anxiety is when you are scared to be around people so being around a lot at once can be a lot to handle

Because it depends on the people themselves. If it was a big bunch of your friends that you are close to you will be alright. But that's a big crowd of people that you superficially know. So you feel shy and you uncomfortable. But when you are with one friend you feel close to and your relationship is good and strong it's natural that you are fine. It all depends on the relationship itself