Each goal can have as many sub-goals as you want and there is no limit to how many levels you can register, but if you have more than 3 levels, it starts to get confusing.
Create objective and sub-goals in the system
To create a new objective, go to the Performance and then Objectives list in the left menu in Manage.
Press the +New button to open the form.
Fill in Title.
If this is a new overarching goal, you do not select an overarching goal in the form. If this is a sub-goal, choose which overarching goal it belongs to.