Strangely worded question. Your game only has one instance, the one you're in.
Other commanders may join your instance, your instance may start in someone else's, you may move into another commander's instance or your instance may calve off and become it's own thing.
ED is a P2P game so the major determining factors are geography, network, and size of the instance. The minor influences are whether you are in a wing or have in-game friends (I say minor but the game will always try to instance you to a friend or a wing).
It's dynamic and changes on major events like jumping, loading, going in and out of SC. If you can be connected to an instance you'll join it, if you can't you get your own. If you live in Australia you'll probably only connect to Australians, if your internet is crap you may connect to no one at all.