I...am not entirely sure I was always in this body from the time it was born, though I have full control over it now, and have for quite some time, and have access to the normal amounts of childhood memories. Kind of like a fully furnished rental unit, and an existence that came with a tutorial mode and HUD interface before the previous tenant took off and left me here on my own, but we were never really *separate* in that one doesn't know what the other is doing or thinking, or can argue with it.
Not enough childhood trauma for a standard split, though I have my doubts about how stable the resulting being would have been by this time had nobody else come along (or bubbled up from inside and made themelves known, whichever it may have been).
I haven't done much research on other walk-ins, I'm still contemplating myself/selves. There's only one being that controls this body at the time, and I can't sort out if memory fragments that do not belong to the body are all mine, or if some are someone else's. So I'm going to say an unusually large value of one, for number present. I don't know if this helps anyone, or if you were just looking for standard named and numbered multiples.
"I" often refers to unusually large values of 1...