I remember reading an interview with Stuart Garner in 2013 where he said that Norton were producing 12 bikes per month, but that shortly after beginning shipments to the US and Canada they would be doubling their assembly staff and doubling production.
So if that was correct, you can estimate the number produced since 2010:
2009 through 2013 – 12 per month = 144 per year x 5 years = 720
2014 through 2015 – 24 per month = 288 per year x 2 years = 576
Estimated total produced prior to 2016 = 1,296 bikes.