"There are a great many windmills in Denmark."
Translation:Der er rigtigt mange vindmøller i Danmark.
I think here the English phrase 'a great many' is the atypical one. Usually you would have an adverb indicating the degree of the adjective/adverb that follows (many). E.g. 'very many windmills' or 'quite many windmills'. It is like this in Danish. There are other adverbs you can use as well, e.g. 'vældigt', 'enormt', 'virkeligt'