There is no "rule" for how many items one may purchase, just as there is no "rule" for many other aspects of how common sense accommodations are made for the situation at hand. Just as on most carriers, the supply of perishable BOB items is limited and it is rare that there is enough to accommodate demand. This FA was apparently more concerned with the rest of the passengers than just OP, so she said what she said.
It was great service on her part to return to OP when she had given others the opportunity to make their purchases.
Carriers limit supplies because they have done significant study of purchasing patterns and the supply vs. space vs. weight issue all boils down to what is catered. One partial solution is AA, which permits the pre-order of certain BOB items. However, the system does not accommodate changes made within 24 hours of flight, whether voluntary or involuntary and, depending on when a change is made, may not make it if there is an aircraft swap.