For a BOB, a complete bolt assembly that you have checked for proper head space is a must. In the event that something on the bolt breaks, just swap bolt assemblies and keep on rockin'.( This can be done in less than a minute with no tools). A spare piston and spring is also a good idea. Rarely needed, but for the money involved, why not?!
Figure out the number of magazines that you will need, add half that many to the number and you will be in pretty good shape.