One of the reasons I think King Harald might be there is that he is Joachim's godfather. Other than that, I don't know.
Møgeltønder church is said to be able to seat about half of the inhabitants of the area, which is about 1400 people, according to information from Felix' christening:
http://www.toender.dk/subhtml/pdf/Ko..._barnedaab.pdf
I don't know how many guests they'll have room for at the private dinner, but I would imagine that that also would factor in with how many royals will be attending.