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. We expect you to be human of course, so the norm we tend to see is one or two concurrent outbound call's per user and then multiple inbound calls depending on how many different sources are calling you.
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.