When I first started working with children with complex communication needs I had never heard that term "Complex Communication Needs". I am pretty sure it was already starting to be used in Australia by then, but it certainly hadn't arrived to my small town in Oregon at the time.
Basically a relatively small number of words (less than 300-500 depending on the researcher) accounts for over 70% (or more depending on the data collected) of what we say. Bruce Baker used this basic linguist principle as the basis of his Minspeak language system that is used on Prentke Romich Company's devices (PRC). He has written up nice summary of core vocabulary with some frequency lists from different researchers here.
For the linguistically geeky, there are many linguists with no interest in AAC who also have collected extensive sets of spoken and written language and counted the relative frequency. Leech, Leyson, and Wilson did one such effort in 2001 and have shared their word frequency lists here. They, and others, have shown that we use different words more frequently for spoken versus written language. Further, when we are speaking conversationally different words are used more frequently than during task based communication.
good, other, right, little, new, nice, big, old, different, sure, sorry, alright, long, great, able, better, local, bad, important, wrong, whole, bloody, only
eah, oh, no, yes, mm, mhm, ah, ooh, aye, aha, hello, ha, eh, dear, bye, yep, hey
of, in, to, for, on, with, at, about, from, like, by, into, as, through, over, after, off, between, against, within, round, under, without, before, across, down, past, during, up, around, regarding
I, you, it, we, they, he, she, them, one, me, who, him, something, us, her, anything, somebody, nothing, everything, everybody, anybody, yourself, someone, mine, myself, nobody, lots, themselves, everyone, anyone, ya, itself, yours, himself, none, ourselves, ours, 'em, plenty, nil, nought, herself, his, whoever, whom, hers, yourselves, theirs
's, is, do, was, have, be, know, got, 've, are, 're, think, get, did, go, had, were, said, 'm, see, mean, going, say, been, want, come, put, has, does, take, look, like, make, doing, done, went, give, thought, need, says, tell, being, thank, saying, 'd, getting, made, find, coming, remember, goes, came, use, having, keep, talking, looking, gone, work, let's, ask, try, let, seen, start, feel, told, comes, wanted, trying, pay, called, call, leave, took, working, left, suppose, talk, am, buy, started, read
can, would, 'll, could, will, can't, 'd, should, might, used, must, won't, may, shall, ought, need, dare