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". You will need a "User" for each connection to our platform. So if you access us from both a hardware phone and a softphone, then we will see that as two separate connections, requiring two separate users. You could of course, disable your hardware phone, then enable your softphone and vice versa each time you wish to use one or another; but you can't have them running concurrently on the same "User". Each "User" then of course has unlimited calling ability to make and receive calls over as many lines as you wish.