Short answer: You choose.
Long answer: All Mint Mobile plans allow you to choose how much 4G LTE data you need each month—3GB, 8GB, 12GB or Unlimited data. When you hit your data limit, we don’t cut you off, we just slow it down. If you need more data, then you can do a one-time data purchase of either 1GB or 3GB in your account (not available on an Unlimited plan). Or if you find you are hitting the limit each month, simply upgrade your plan to the next data level at any time.