My daughter is an only child but has eight cousins (that I know of, most are in her age range) but barely spends time with them. She's not missing out on anything because she has a large circle of friends her age, be it from the public playground when they only started walking, kindergarden or now the first three years in school. It takes some effort as a parent and tbh dumping-the-child-at-her-friends-and-vice-versa but she's got a much better social life than I had at her age and she'll continue that even though she'll move 4hrs away to the country-side this summer. Her best friend actually moved away two years ago, we still make sure they meet up and have sleep-overs and such.
Lots of people here write about the number of cousins, but those are often of a different age, it was the same with my two cousins. Also missing from the article: Kids being pushed into extra-curricula after school, be it soccer or music, those might not necessarily lead to friendships? Lastly, our environment has changed a lot. It's just not safe to let kids run wild outside with all those cars. The number of kids killed by cars didn't sink over the past 40 years because cars got safe, it sunk because kids get basically locked in. In my city (in Austria) we have quite a lot of older and newer building complexes where cars are kept outside, those are much better for kids.