As said above most units share the same number if chartered by the same org. I do know of 1 unit in council that has the same CO and their female troop has a completely different unit number so I am assuming multiple unit numbers tied to a CO are possible but probably not preferred.
As others have said I also have the understanding that units are actually 4 digit in the national system; however, I think that the leading number could be different from council-to-council due to mergers. My council has a few troops with the same unit number because of council mergers. My understanding is that when the councils merged every unit that conflicted received a leading number if they were newer than the oldest conflicting unit (EG: 0222 is 50 years old, they didn't change, but 0222 from the other council was only 12 years old and became 6222 in the system and both are allowed to wear 222 on their uniforms). My council seems to have applied a 6 prefix to one former council and other prefix numbers to units from other former councils, and a third from the absorbing council; the oldest unit regardless of original council always gets first dibs on keeping the leading zeros.