Each Lightning Payroll annual subscription is able to be used by two computers (Windows or Mac) at any one time. The data is not automatically shared though, as Lightning Payroll is a desktop application, but it can be easily transferred via the numerous backup methods, such as our Online Backups feature.
If you have a pc which will only be viewing the payroll data, and not editing, a licence seat is not required. See here for more information about the three licence 'modes' of the software.If you need more than two systems using the payroll at any one time, you can simply purchase additional annual subscriptions under your account, and each will provide you with two more seats/installs.
Please see here to learn about moving seats/installs between pcs.