How many calls can my 03 number take at once?

That depends on the SIP lines on your FreeVoIP account, not on the 03 number itself.

A free account includes two SIP lines (two simultaneous calls). For a busy national number, add more lines from My Account, up to the 25-line SIP account at £25/month.

Set your line count to cover your busiest expected minute, and route any overflow into a call queue so extra callers wait with music on hold instead of getting an engaged tone. The 03 number never changes — you're just adding capacity behind it.