Multiple Choice fields allow for a user to select between pre-existing answers when filling out a form. There is one multiple choice field type that allows for more than one answer to be selected: Checkbox.
To add a Checkbox field to a form:
In the Web Portal go to Forms > Build New Template (if creating a new form template) > enter details and save > Form Builder;
or Forms > Existing Templates > edit arrow for the template > Form Builder tab.
You can add the field by selecting Multiple Choice in the Toolbox, and then dragging and dropping the Checkbox field where you would like it to go in the form.
Once you've added the Checkbox field, select it and the Field Properties will appear on the right side of the Form Builder. Here you can edit the field and how you'd like it set up. Once you have, then Save Draft and Deploy / Publish Template, and the Checkbox field will appear on new forms that are created with this template after the changes were made, where you will be able to select more than one answer.
(Note: These changes will not be retroactively added to any forms that were started before the template was changed, only newly started forms.)