"One of the cats sleeps."
Translation:En av kattene sover.
Both 'man' and 'en' can refer to a general person. 'man' can however only be used as a subject in such sentences.
Both 'en' and 'én' can refer to the number one. Depending on the context one will be better than the other. 'én' is often used if it's important to express 'exactly one', but accents are always optional. In this context it's not really important to be exact, so 'en' is used.