I have been to so many types of christenings ranging from huge gatherings of family and friends almost filling a church, to small intimate celebrations with no more than parents, godparents and grandparents there.
It all depends on your views about christenings, family dynamics and taste I suppose. I don't think there is a specific etiquette.
My DD christening had 50 people invited. We kept it to closest family (who had taken the time to visit our DD) and friends who likewise demonstrated interest in our little girl.
We were quite ruthless in our guest list. It was our belief that since the celebration was for DD, only those had been part of her little life so far should come.
The only exception we made was for a lovely family member who lives in Scotland and who had not met our DD due to distance. She did travel down to the christening though.
I have known christenings that vary massively from just the closest people - parents and godparents - to whole family and friends affairs.
The christening for my own DD was a big one with many guests including friends and family. It was a service just for us, no one other than our party of people. Then we had a lunch and celebration afterwards. We used it as not only the baptism but also a celebration. So we had probably 60-80 guests I would imagine, maybe more. We invited same people as we had our wedding pretty much.
to a Christening? I'm sure people do all sorts of things, but what is the norm? I don't know where to draw the line from one end with only parents/ grandparents/ godparents to all distant relations and acquaintances.
What's normal?? Or your own experiences all helpful!