Continue in Stockinette, dec 1 st at each armhole edge every other row 4 (5, 7, 8, 9, 11, 12) times.
Does it mean at eachedge I decrease 8 times, or does it mean decrease at each edge and then don’t decrease for hte next 2 rows, and then decrease the next 2, etc. ?
Decrease 1 stitch at each edge every other row, 8 times.
Row 1, decrease
Row 2, don’t decrease
Row 3, decrease
Row 4, don’t
etc until you’ve decreased on 8 rows. 
I would say on the rs, you decrease 1 stitch, knit across to the last 2 stitches, decrease on those. Then turn and do one row of purl. That counts as one. You do that 4 (5, 7, 8, 9, 11, 12) times.