Are knedlíky something one orders a specific number of? I had thought they were quite small, like Austrian spätzle or Hungarian csipetke.
Nope, they are not similar to spätzle. They are boiled as a whole and then sliced. The number refers to the number of 'slices'.
Six sounds about right, to start with anyway. Does anyone eat just one dumpling?
In some Czech/Slovak restaurants in the U.S., one does get just two by default. If memory serves, four was the default in the old country. "Maďara s šesti" was my favorite off-campus lunch order. Those were the days...