To set up and configure, read the doc: https://www.odoo.com/documentation/user/9.0/point_of_sale.html

You have to use 1 posbox per station, not 1 posbox per store. This means that if you have 3 stores with 4 stations in each, you need 12 posboxes and you will have 12 records in your list of POSes in Odoo. Yes, you can have all this in 1 Odoo instance.