My DS is 7, in year 2. He is almost certainly neurodivergent (we think inattentive ADHD, also possibly ASD). I'd describe him as introverted but pretty sociable and on the outside often seems confident, loud etc, albeit quirky. He's sensitive, kind, funny and diplomatic. He would never voluntarily play on his own beyond toddler years.
He's always had a small number of friends and seems happy with that. In private day nursery, it was him and two other boys. In pre-school he became close with one boy, who is also loud, confident, a million miles an hour, and they would get up to mischief together. He has stayed friends with this boy and in reception made two other close friends, one boy and one girl. For a while the four seemed to play together a lot but this fizzled out and it now seems to be the three boys, or maybe two of them. He still likes the girl and will play with her at parties or on play dates, but I get the impression they don't play together much in school any more.
He doesn't branch out beyond these friends, at all. He is getting slightly better at working with other children in class, apparently, but will always prefer to work with his friends. Without fail at break times it's the two boys he plays with (often in a threesome, sometimes with one or the other). His teacher mentioned at parents' evening that the 3 boys seem to play role play type games more so whereas a lot of the others play traditional tag, chase (which DS does like but I can see why role playing with your besties is more fun). It's a 3 form entry school so plenty of other children too.
He goes to clubs without the friends, and also after school club once a week, and I don't get the impression that he's ever alone without anyone to play with/sit with, but I think at these clubs he'll play with whoever is doing the same activity that he wants to do, rather than making friends.
Should I worry? I don't know how normal this is. I only ever played with the same two girls at primary but I think I might also be ND and also my school year was very small so there was a limited pool of friends. I have held a couple of play dates with children outside of his little circle, and they get reciprocated but as the children don't really seem hugely close nothing has ever come from it. I'm worried he will be so used to only having these two friends all through primary that he'll struggle when it comes to secondary.