I've always read eight, though the missiles might never have carried that many as it would have an effect on range (W88 is significantly heavier than W76). In any case, the Trident force currently is probably more likely loaded than that, given that there's about ~900 warheads reported deployed split among 240 declared launchers (12 x 20 tubes in deployed status at any one time), so the average number of warheads per missile would actually be ~4. How exactly they are loaded is of course unknown; it is likely that W76 mod 2 missiles carry only 1-2 warheads and that other warheads carry more like 4-6 depending on the envisioned target set. W88 makes up roughly 400 of the deployed warheads with W76 mod 1/2 representing the rest.