Depends on the filter from Hoya. Some are designated NDx2, NDx4, NDx8 and those indicate exposure factors. Others are designated ND03, ND13, ND25, ND40, ND50, ND70, and those indicate percent transmittance, as in 3%, 13%, 25%, 40%, 50%, and 70%, but I think those are mostly marketed to the scientific community. I haven't seen Hoya filters primarily designated in stops or with 0.3 units/stop numbers.
BHphotovideo shows a Hoya NDx400 (2.7 density units, or 9 stops) in 72mm for US$125.00. Denser ND filters are usually rather expensive because it's difficult to make a filter that dense with uniform transmittance across the photographic spectrum.
B+W used to make 13 stop and 20 stop ND filters, but I think those are out of production.