Ok. Most people use the term alters, I tend to switch between this and Identities, because I prefer the terming of identities, but professionals talk to be about alters.
There is Me, plus 8 identities (at the moment; it used to be less). Usually it is only two that are prominent, although the others are there, I don't think they surface very often but I am very aware of them.
Having 8 doesn't mean that for 90 oddish percent of the time I don't remember anything; there's no fixed pattern related to memory versus the number of identities.
I should preface this all by saying this is obviously only my experience and I am sure other people will answer your questions differently, particularly around alters/identities.