Why not buy something like the juices they have at Trader Joe's (they come in tall, slightly fancy glass bottles) and offer them with or without the prosecco? I think there's an OJ one, a pomegranate one that's good, and some lemonade varieties.
How many bottles depends on how much you expect the people to drink. In my circle I'd buy a bottle for every two people at a minimum, so at least ten. Typically people drink less of sparkling wine drinks than other wine because unlike red and white wine glasses, champagne glasses have remained about the same size. Of course, I serve mimosas with a touch of oj and mostly sparkling wine - about like an orangina. You can reduce the amount of alcohol by serving them half oj half wine, but I think that's too sweet.