I would think the number of backup systems may also be dependent on imperial laws. For instance, within the Terran Star Empire (TSE) of my sf game, they require passenger liners to have at least 2 full life support backups and at least enough lifeboats/lifepods for half again the number of passengers and crew.
However, ultimately, it is the size/volume of the ship that will dictate how many backup systems you can have on a ship. In the TSE Naval forces, frigates are so small that about the only backups they can afford is 1 extra life support and 1 auxilliary power generator. The larger battle wagons such as dreadnoughts, battleships, etc. can afford more backup systems along with workshops to make repairs.
Basically, as Corey says in his post.
One hole in the TSE laws is with freighter ships. They are mostly concerned with being able to haul the maximum amount of cargo rather than worrying about the crew.
rmfr