I recently provided restoration services for two sets of original 46IDA3C Webers from two different 911Rs; these were for cars #11 and #19.  I used this opportunity to document the subtle differences between these race application Webers and those used on production 911s. 

The engines for the 911Rs were essentially the same as for the first 906s (904s with 6 cylinder engines dating from 1963) and later the Carrera 6s (906 designation again).  In round numbers there were 60 six cylinder race engines mated with either 904/6 or 906 chassis; it then makes sense that Porsche would purchase a convenient quantity of carburetors to fulfill the production requirements along with sufficient spares.  I assume a buy quantity of 75 to 100 sets as a reasonable guess.  The Webers I serviced all had one or two digit serial numbers, the highest number being #46.