More than 13,000,000 cars were built on the 65-70 GM B-Body platform. It is the third best selling platform of all time behind the Beetle and Model T. There's a Lada up there too but I don't know how Russians count cars!
I read somewhere years ago that 1965 was the biggest selling year for either impala or the B Bidy platform in general I think. My impression was that the intermediates and pony car started sucking up sales after 1965.
That would make sense... but if you look here, from a quick scan, almost seems to peak in 1969. Someone needs to add them up, to know for sure, as they change model summing year by year...
I'm quite sure Chevy fullsize production peaked in 65. The only two stats I have handy are as follows; total fullsize chevy production in 1969 was 1,168,000 but in 1965 JUST the Impala was at 1,047,000. Add to that Caprice, Biscayne and Bel air you are much higher than 69.
1965 was a great year for the auto industry, with 1968 another good one, 1969 was a bit soft industry wise.