I was recently looking at the akl-Bkk for a trip a few months later than yours and couldn't find multiple seats across a variety of dates. I ended up going through bne at the cost of an extra 8 hours total travel time.
That said, I was able to book 4 C seats from bke-bkk using the approach outlined in this thread- booked 2 and the other 2 showed up immediately.