The deal is good on either a small Chocolate or Vanilla Frosty, which is normally $1 or more, depending on your location. There's no limit on how many you can buy.
Additionally, Wendy's has partnered up with MLB.com to give a free subscription to MLB.TV Premium for the rest of the 2016 season (through October 2, 2016) to the first 34,000 fans who post a selfie with their Frosty on their public Twitter or Instagram account using the hashtag #50centFrostyMLB through the end of the month. You can find further details on the promotion's website here.