It's a lot. Us parents joke how insane it all is but realistically it will taper off soon as kids start having smaller birthday celebrations. At this age it's kind of a "invite everyone in your class/grade" and has naturally reduced a bit already as boy/girl only parties started. I think next year or two it will become more common to have "invite 3-5 good friends to an event" type of birthdays and that will reduce it a lot further. Usually that's also the beginning of "drop-off birthday parties" where us parents don't have to attend with our guest. There was only one this year, my son was picked up and a group of ~10 went to a sport event.

Oddly enough, there are practically none in summer. If you have a summer birthday you either don't have a big party or you have a half birthday or something similar where the party occurs during the school year. Too many people travel throughout the summer and kids are doing different camps and things so it would not get well attended. Our group of parents kind of have unspoken rule to not do anything that feels required when school is out. That goes for fall/summer/spring breaks and holidays too.

The logistics part probably sounds crazy but probably only ~10% of these parties are at someone's house. We've never hosted a party at our house, well when he was 1-2 for family only, but not these huge parties with so many kids, parents, siblings, etc. Most people rent out a venue. Arcades, trampoline/slide parks, skating rinks are popular with the girls, sports themed places are popular with boys, chuck-e-cheese was popular for a bit, those kinds of things. It's too much work for a 2 hour party to have that many people in your home.