Is he your boyfriend or your 60 year old mother? Keep asking you to go out more and hang out with friends lmao. It seems like he values people based on how many friends they have.
I believe you should tell him:
1) It is by choice that you have this few friends. You don’t need more because you’re perfectly happy with what you got.
2) More friends doesn’t equate to more value. Your value will not diminish just because you have only a few friends.
3) If he enjoys meeting new people and changing friends every so often, that is his choice, you might not agree with it but it’s not like you keep asking him to cut down on friends, right? So why is he telling you to get more? He should not impose his lifestyle on you.
If he refuses to understand you then you should reevaluate your relationship, later he replace you with someone more “popular”. As an introvert you cannot just suddenly enjoy company. If you submit to him and start “going out more” and “finding new friends” just to please him, your life is going to be very tiring.