Translation:I want a million people at my wedding.
First of all, you wouldn't say "in" my wedding. It's "at." People come to the venue, they cannot get "in" to a wedding. Perhaps you might say that if they were actually participating in the service, but there can only be so many priests and bridesmaids, etc., never a million people!
Then regarding "one million" -- English speakers will invariably say "a dozen, a hundred, a thousand, a million" etc. because it's easier to pronounce. It also carries a semantic implication that the number might be approximate rather than a precise count. Of course, it is not incorrect to use "one" in these situations and Duolingo should accept it. In fact, if other numbers follow (implicating a precise count), then we would probably say "one." For example, a radio announcer might say, "we have reached our goal of one thousand five hundred dollars in donations today."
Use "di" whenever you need the indefinite article. A dozen, a hundred, a thousand. Italians also use the word "decina" for when you have about ten of something. So you would say "Voglio una decina di caramelle." Interestingly, they count and sell eggs in dozens just like us.