How many lines do I need?

Every VoIP account comes with unlimited lines, so you simply don't need to worry. Unlimited basically means that you can have as many outbound or inbound calls as you wish on the account. 

In terms of users, each user can have 1 concurrent outbound call as part of their inclusive minutes plan, any additional outbound calls will then be charged on a pay-as-you-go rate at the rates shown here

Lines are different from Users. Each User can have up to two handsets registered, as well as a softphone on a mobile, and a desktop/browser app connection. These will all run concurrently, and unless set to Do not Disturb, any inbound calls to your User will ring on all registered devices.