The till screen isn't a separate program; it's a page in the browser. The tablet, laptop or PC you already have is enough.
Every cashier has their own account, so every payment and refund is attributable. When staff change you close the account and nothing lingers on the device.
Cashiers only take payments; pricing, campaigns and reports stay with the owner.
Each branch's transactions are recorded to that branch; nothing gets mixed up.
No limit. Add as many cashiers per branch as you need.