Just like any other model kit manufacturer Wingnuts are not going to tell you how many kits they have sold, try writing to Tamiya or Eduard and ask them their sales figures, I'm sure they will be as silent as Wingnuts.
Many years ago I read a report "somewhere" that a scientific study had been done on the number of modelers who actually make WW1 aircraft models, the figure was surprising, 10,000 approx. Bear in mind that this is worldwide so as a percentage of population it is a very very tiny figure. Forum numbers would not give an indication of WW1 modelers, I know many modelers who have never and never will join a forum but still build WW1 model aircraft. Even the 2000 + members on this forum don't all build models, many have never left a post since they first joined.
Wingnuts are doing well, they are selling quite a few kits and will continue to do so until such time as SPJ decides he doesn't want to play this game anymore.
Getting back to the original question, I stick with my first answer, I wouldn't hazzard a guess - but I am very thankful that Wingnuts are around regardless of how many kits they sell.
Des.