It depends on your plan:
With the free (prepaid) account, you can make one call at a time. One inbound channel is included for SIP calls (ippi users and SIP addresses). To receive calls on a number, add at least one inbound line.
With unlimited plans, each plan adds one simultaneous outbound and one inbound call. For example, with 3 plans you can make 3 calls and receive 3 at the same time.
To make more calls at the same time, add a plan. To receive more, add inbound lines from your dashboard, without taking another plan: with 2 plans and 3 inbound lines, you can make 2 calls and receive 5 at the same time. There is no limit on the number of inbound lines.