like bob posted, 8 and under hmpe fiber is considered skinny, 10 and over ain't.
this is for most hunting stick bows in the 40+ to 70+ range. yes, 8 strands of hmpe is more than enuf with 70 pounds holding weight, considering a single strand of the average hmpe has over 100# tensile strength, which will yield more than a 10:1 bow:strand ratio.
there are no rules, do yer own testing and see what works best for you. that stuff can be fun, particularly if you can twist or spin up yer own bowstrings.
6 strands or 16 strands, it's all good if it works good for you and you know what yer doing.