Its a variable. Dependant on your pipe, port timing, power valve, compression ratio, carb size, how clean the casting is in your intakes/transfers. I would say 10 would be the upper spec and likely into over rev where the power will be tapering off.
Mine tapers off before that, and just stops at 9. I know it hasn't been rebuilt for quite a while before I bought it, so a full engine rebuild will happen later this year. I'm new to Gassers so was just trying to get an idea of how a good one performs. I've spent a few weeks lurking on here, so thanks to advice given to others, I've a fairly good idea of what to check for.