Can you have multiple Spectres following you by hacking more than one? If so, is there a limit on how many you can have at once? Can I realize my dream of creating my own private army?
the only limit is how many there are in action at the time. You could in theory hack every last enemy spectre. This is quite good on one of the campaign multiplayer maps (no spoilers) as enemy spectres are every 2 steps.
Yes, you can have a spectre army. Whether or not they do anything productive is besides the point.