Why do I have so few friends?

I am a mid 20s female and I have like, 4 friends and I feel like something is wrong with me or I am missing something. Am I supposed to have a lot of friends? I feel like it is abnormal to have so few. I have had a few more but we don’t talk anymore. I am a friendly person, I have never been bullied or mean to people, I don’t have a mental illness like autism or bipolar, etc. Why am I so socially isolated? I don’t want to have a large social circle like 50 plus friends, but I feel like 4 is too few. I feel like I am missing out on life. My birthday is in a few days and I don’t know if I am going to celebrate or just spend it alone. I would invite my friends, but I feel like they don’t want to celebrate my birthday. My family would throw a party for me but that is just embarrassing. I am an adult, I don’t want my family throwing a party for me and it would be for kids. I am a positive person but this is something that has been weighing on me for literally years. I just want to know what is “normal.”