How many calls can my DID handle at the same time?

That's set by the number of SIP lines on your FreeVoIP account, not by the DID.

A free account includes two SIP lines, so two simultaneous calls. If your DID needs to take more at once, add extra lines from My Account, up to the 25-line SIP account (£25/month) aimed at offices.

A good rule of thumb: match your line count to the most calls you'd expect ringing in the same minute. Anything beyond your line limit can be sent to a call queue or voicemail rather than getting an engaged tone.