With so many system members, how do you keep track?
Short answer: we don't.
Slightly longer answer: We don't really have to, because we're all here anyway, to remind each other of our existence.
Longest answer we'll give you right now:
We're like a very large city or a very small country in size. Unlike our outworld counterparts, though, we do not have the bureaucratic structure or the technology necessary to create a directory of everyone in our population. At least, not one that is accessible from outside our system. We have our brain, and it's natural networks of neurons seem to do a pretty good job anyway.
To pull a system member forward, we just need to have a memory or a sensory stimula they associate with, such as their name. It doesn't always work, because they are not always awake. However, this is much like an outworld city. You might learn someone's phone number or email address, but they don't have to answer.
And just like in an outworld city, where not even the mayor knows everyone's names, no one in our system has the memory capacity to learn about everyone who exists in our head.
But we do have system celebrities and prominent figures who front regularly or play roles in our media (such as in our comics or social media posts). Also, we have this wiki and our autobiography. These tools are useful for confirming to the outworld that certain members do resurface often and that they are the same people they were before. And these prominent system members give us a sense of a sort of cultural or system identity.
Still, we have so much traffic in our head, and switch who is fronting so regularly, that no, we cannot keep track in any practical, useful sense to the outside world.
At least, not beyond what you see as evidenced in this wiki.