Well, you have two different things going on. There's the simple indefinite article which is et for neuter, ei for feminine, and en for masculine. Then, each of those is intensified, as it were, to mean a quantity of one. For et, a second t is added: ett. For ei and en, an acute accent is added to the e: éi and én.
It looks like "en" can mean "a", "an", "one"..., but " én" can only mean "one". http://www.norskengelskordbok.com/en/dictionary-norwegian-english/%C3%A9n