Then you spawn a few dozen of them in, just for testing purposes, and try to command one to raise their arm by teleporting an armor stand CustomNamed mecharightarm up a bit and... every single one of them raises their arm.
This is the kind of a problem that I ran into today, and a lot of times before when commands weren't as advanced as today, and just can't seem to be able to solve.
The logical solution is to break the entities up into separate sets, for instance by giving each armor stand an ID with scoreboard. But then, how does one select entities that have scores equal to another entity?
What are the solutions to running commands on sets of entities rather than just one?