For a number of years, from the restart of the Isle of Man TT, after WWII & so that would be '47. I would say 1 to 7 are killed almost every year. Considering the amazing speeds now compared to to the 60s when no one had done one lap at the 'ton' or 100mph & now that is so common to even 130 plus mph. True the bikes were 125, 250, 350 & 500 bar a few like 1000cc Vincent HRD. Drum brakes & all in those old days.
Reason the Isle of Man TT was taken off the World Championship series back in 50s of some time. That is one deadly 37.75 miles pr lap & to many first time competitors were those to be killed.