In Kindergarten... kids do not have "BFF's."
And, friends are not static nor the same every day.
Many don't even have "friends" yet.
They are young... kids play with whomever or themselves.
At this young age, friendships are not cemented yet... nor fully formed.
It is not, for all kids.
My daughter was like your daughter. But at the same time, my daughter even if she was "shy"... stood up for herself and was very self- aware and self-reliant and self-assured. She didn't race, to be friends with others or have friends like others or keep up... with other kids.
She was... herself.
So, I did not get flustered about whether or not she had friends or not or how many. Because... my daughter, "chose" her friends. Instead of visa versa. And she did it consciously, if she was comfortable with or trusted a child. She wasn't a follower, and actually abhorred "bossy" kids.
Let your daughter be herself. Teach her to be herself.
And let her know... that kids this age... do not have consistent BFF's or BFF's at all.
At this age, kids are really just still learning about socializing and friendships. They don't know, everything yet.
Your daughter is fine.
Being a friend and having a friend... means, teaching and guiding your child... about how to "choose" friends. How to "discern" other people. So they do not... get wrapped up... in following others or parroting others or just going along with others, because the other kids are more extroverted.
Teach her... how to be herself.
That is what I always teach my kids.
And by chemistry and like mindedness... a child, will find their way and to friends... who compliment them. Not overbearing, to them.
Teach and guide... your daughter about these things.
That is what I did with both my kids... from when they were very young.
That will teach her about choosing friends, and that, she is not just marginal... to other kids.
It will teach her... how to be, self-assured.
And that comparing herself to others, is not what matters.
Both my kids... are not super extroverts, but by teaching them these things, they are very self-assured, choose good friends and are not caught up in "how many" friends they have, versus other kids.
But... they do have friends... GOOD kids who are good friends. Friends that are not overbearing to them. But comrades.
That is what, a child will best do with.