My DC is 6, in year one, and has had the same close friend since the nursery class. He also has two other close friends that he made in reception, and in parents' evening when I asked about this, his teacher said he tends to only play with these three children (sometimes all at once but sometimes 1-2 at a time). One of the friends dips in and out a lot whereas DC and the other two are in a bit of a trio, which I know can be an issue (and his teacher did make this point - "three's a crowd").
My DC claims that he does play with other children at the same time as well as these three, but his teacher said not.
We discussed encouraging him to play with others as well, which I'm trying and agree with as it's a shame not too when the school has such a large pool of playmates (3 form entry) although it's tricky when DC tells me he is playing with others!
But I wanted to gauge from parents maybe with experience of this whether it is, in fact, a problem? My DC isn't the most socially confident child but if he sees a child he knows outside of these three close friends, whether it's another from his class or from one of the other classes (big school - 3 form entry) at the playground, or at a club, he is sociable/chatty enough. I don't worry about him being on his own as I can't imagine, if the three friends were off sick for example, that he would spend all playtimes on his own.