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

I feel the same way! Sometimes it may be easier to express ourselves with an "audience of one" than with a bigger group because we can better judge how that one friend will react. It is a completely normal feeling but personally I have benefited from exposing myself to bigger crowds (in baby steps!) and it has helped my social anxiety a lot.

Because you may not know if they will judge you. I do not talk in big groups only one on one. And that is okay

It can be much easier to "read" one person, both in body language and in verbal tone, in order to feel comfortable with them and make a connection with an individual. Large crowds, however, do not have a singular individuality to connect with and thus can be more intimidating, but remember that they are made up of individuals just like you and relating to or with them can be exactly the same as to one person to help ease your stress level.

Big crowds usually make people more nervous because there are so many people versus just one person. In addition you know your friend very well meanwhile a big crowd is full of strangers, who people are usually less comfortable with.

This could be social anxiety it's where your really awkward when your in big groups of people I recommend you talk to someone about this x

You may have an introvert personality type. The more you practice overcoming your shyness by meeting with larger groups, you will feel desensitised the anxiety that holds you back

Usually when you are around strangers you feel scared as you don't know them but feel fine with a friend because you know you can trust them with anything.

Honestly at least I know for me it's because I feel like they are judging every move I make and I can't explain myself. But when I am with one friend I can focus on them and I feel like I have more support.

It may sound like you have a co-dependent anxiety, when alone and around large groups of people, you feel as if the walls are closing in and worried that you're being eyed down by everyone. When you have that friend it takes away all that anxiety because you're not doing something alone and not feeling singled out, you have that presence of support. Does this sound Viable to you ?

Well....that is a symptom of social anxiety but I will not jump to conclusions and say that you have socia anxiety. Maybe it’s because you’re quite introverted and you’re only comfortable talking to one individual rather than talking in a crowd, because that’s quite nerve racking