The plural of 'eventyr' is same spelling 'eventyr', isn'tit? But the plural of 'the adventure = eventyren' is 'the adventures=evntyrene', wright?
It is eventyret singular and eventyrene in plural. You are right eventyr itself can be singular and plural.
Btw, it's always a good idea to check ordnet.dk for such things, it sums up perfectly what you needed. :) It's such a great resource, you should check it out here: http://ordnet.dk/ddo/ordbog?query=eventyr
I need to search back in previous lessons to know the difference between 'where' and 'how'. :( Although probably a 'how' question regarding the adventures would look different in Danish.