Building an email list can be difficult unless you can offer something enticing. Sweepstakes are perfect for building your list. You definitely need to have specific prizes and freebies in mind before starting it. It is not the quantity so much as the quality you want to have. Be very precise in how many prizes and how it is awarded when you send out the newsletter to your subscribers.
The prizes need to be something your target audience will be attracted by. This gains their interest in emails from your company. You want people who will read what you send, not sign up for the sweepstakes and then unsubscribe the minute they receive an email.