I don't always "have to have" a person, but i definitely do better when i have "my person" in any given setting, ESPECIALLY at parties. I don't like to go alone, i need the security of "my person", like you said. Some days, I am able to branch out and be super social, and other times I get freaked out and just have to stick by my person, but either way, I feel much more secure knowing that i have a "person" at the event with me. It's sort of like having a home base, or the spot that's "safe" when you play tag. "My person" is someone who knows about my anxiety and social difficulties, even if they don't know about my ASD, and they can at least understand when i'm having trouble.
For my life in general, my best friend/sister is MY PERSON and i feel infinitely better when she's around no matter the circumstances. I would always choose to have her with me over anyone else, but it's not possible for her to be around all the time now... in her absence, i've noticed that i do tend to attach to one person in a friend group, work or school setting, etc. It has to be someone who meshes well with me, so i don't always find someone to bond with and be "my person". When that happens, my anxiety is definitely higher and i definitely miss out on more activities and fun. I've never thought to think of it as a "security blanket", but I guess it kind of is.