No you only need one coupon (as long as you aren't stopping over at the intermediate point), but can split into 2 if you choose. If you want to make good use of all segments this can be highly advantageous (eg LHR-AKL using only one coupon). The sector count depends on if you have 1 or 2 coupons on your ticket.

Generally FFPs credit only per the distance between origin of first leg and destination of last leg, ignoring the stop in between. Ie KIX-BOS in your case. For some flights there is very little difference in total mileage (except perhaps minimum miles if one of the legs is short), but for others there could be a considerable difference (eg SIN-BKK-NRT is much more miles as 2 separate flights than as one direct but with a stop flight).

So there are advantages and disadvantages of 1 or 2 coupons, and these depend very much on your FFP, routing and circumstances.

Note that some airlines may be reluctant to ticket as 2 coupons for a single flight number if you aren't making a stopover en route.