agree with everyone else - smaller parties are find. The general rule I went with my kids was either whole class party or all the girls/boys or less than half the class or boy/girl group. That way you didn’t end up inviting 9/10 of the girls and leaving one out.
the problem with only inviting the 10 that invited you to thiers is
children 1-5 - still friends with all good
child ren 6-7 had whole class parties and actually you son doesn’t ever really speak to them at school
child 8 - they were friends 10 months ago but no me this child is excluding yours from playtime and being mean
child 9 - has moved away
child 10 - only invited yours as you invited them last year
meanwhile
child 11 has only moved into the class so hasn’t had a birthday yet
child 12 has parents who don’t do parties but do a family day trip instead but is really good friends
child 13 is extremely poor so can’t afford thier own party
so you method gives your son only half the people at his own party who he is good friends with and potentially leaves out some he really enjoys spending time with.
younare thinking with your adult head because in adulthood yes we can invite thoses eg to our wedding that only invited us to ours as because we are adults we have control over who we interact with and know those people didn’t invite us because they didn’t want to. Children like your son are at the mercy of thier parents decisions so will miss out through no fault of their own.
let your child choose who he wants. It’s often one of the only things they get control over! And is a good learning curve on deciding who to invite.
NB I do sometimes intervene if the guest list is going to leave out someone eg only one boy not invited etc