Final answer:
A flag officer or unit commander afloat can display their personal flag or command pennant from only one vessel at a time, according to traditional naval protocols.
Step-by-step explanation:
A flag officer or unit commander afloat can display their personal flag or command pennant from one vessel at a time. According to traditional naval protocols, these symbols of command are meant for the singular flagship or the vessel where the officer is physically present or the one that is currently acting as the command vessel in their absence. This means the correct answer is:
1) 1 vessel.