It's 12 GB per month, no rollover, reset each month. I've had pretty good luck with this company. I ported my landline to them with no problems and I just added another phone to use as WIFI hotspot. For these I use an unlocked Samsung (J3 and F7). No problems.
For activation, put you phone on WIFI if possible, enter you activation code from the sim into mintmobile.com/activate, they seem to find the phone over wifi, done. Remember to switch off wifi and you will be on mobile.
They're the cheapest I've found for a lot of data. If you want unlimited data for more money you can try calyx institute.