Everyone has a best friend or multiple. As the saying goes, quality over quantity. As we get older, that group of "best friends forever" that you swore you would stick with throughout your life slowly dwindle down from that group of ten to a group of two. Often, we have that one person who has stuck by us through thick and thin and has overall proved their loyalty and devotion as a friend. There may be many people in your life who are like that. You may refer to all of those loyal people as your "best friend."
You can have more than one best friend.
A lot of people don't know this. Some friends get jealous when you call someone besides them your "best friend." Many people don't understand this. I have six best friends and I refer to them as my "best friends" because I consider them to be the best out of all my friends. That's what a best friend is, isn't it? So why can't you have more than one?
Getting jealous over friendships is so "first grade."
You don't have to be number one. There may not even be a "number one." Getting jealous of your best friend's best friend is something we got over in first grade. This isn't a competition. You don't receive an award for being number one. As long as this friend is a loyal one, who cares if they call other people their "best friend." Getting jealous over something so juvenile only ruins your friendship, not the one you are jealous of. It does nothing but hurt your friendship.
So I'm going to say it one more time just to clarify: you can have multiple best friends. There is absolutely no reason for jealousy. Trust your friends, respect your friends, respect their friends. You don't have to like everyone your friend associates with because you don't have to like every single person you come in contact with, and most often, you are going to dislike someone at some point. If your friend is happy, their friends shouldn't matter to you because you are still in their life and you are still their best friend. Drama is overrated - no need for jealousy to cause drama.