Upon creation of a profile, yes, there is a single box for sources, but that single "box" or window can contain many sources.
Your comment is the first one I've seen that has interpreted that single box to mean "place only one source here."
Many of us use inline source citations to assign specific sources to specific data. But one could use that source box (at profile creation-- later, post-creation, biography and source boxes are contained in a single narrative "box" or window) to list, starting with an asterisk at the beginning of each line sources like this:
* Birth: Certificate of Birth - John Smith (Buffalo, Erie, New York; 12 Feb 1924), certificate #12345; copy held by Jillaine Smith (address withheld for privacy)
* Marriage: Marriage certificate of John Smith and Miriam Sess (Buffalo, Erie, New York; 22 Jun 1946); certificate #987654), etc...
* Death: Certificate of death... etc.