How do you get farms in addition to your current shelter?

A farm is just another game account. You will need to either start a game on a different device or make sure you have bound your current game to an external account (recommend using the IM30 option) and then start a new game on your current device. When you start the new game bind it to a different IM30 account. You can then swap between them. We have a guide page for creating a farm with more details.