I sit here trying to think of a snappy answer, but there isnt one. Aircraft have evolved over time.
The B747-100 had INS which were big boxes of gyros. They only provided navigation. The ADIs were driven by their own gyros.
The Tristar had positions for three INS, but we only had one fitted on ours, and two racks each with a VG and a DG. The INS drove the ADI on one side, and the VG DG drove the other.
The B737-400 had TWO INS, each driving its own ADI, and able to feed both sides. There was no spare. There was of course a standby ADI.
The B777 has a single INS (with six gyros inside it) and a SAARU (standby unit with gyros. Either box can drive all three ADIs.
I am sure there is something better in the B787.
I would appreciate it if someone could correct me if I am wrong . I am an A and C guy who spends his life on the ramp.