Thanks!
How many ceremonies are held depends on the Gemeinde. It seems that even big ones such as Zürich hold just one each year, but that need to apply to all.
With that said, do naturalised people swear, perhaps allegiance to the constitution? I would think only soldiers do as well as certain federal kantonal and federal employees, but not ordinary citizens-to-be.
What I would expect is a small festivity, an apéro if it's held during the day or a decent dinner in case the event is held in the evening, with a Regierungsrat or two attending, perhaps a short speech beforehand. But not an oath or similar (I am known to have been wrong though) - the Constitution and everything stemming therefrom serves you the (new) citizen, not the other way round.
In the case of Zürich city, of about 3'000 naturalized persons about 1'700 attended the Einbürgerungsfeier last year, so it's not mandatory.
There was no ceremony, no swearing in - also no language test