Earlier quoted context omitted.
Try Google. This is pretty widely known phenomena, as in anyone who has read anything about cold war espionage is aware of it. This article confirms it through a source from the former KGB. Here's a 1985 NY Times article detailing Soviet use of this ruse in Mexico: http://www.nytimes.com/1985/06/23/world/mexico-city-depicted... Here's a 1983 NY Times article "Soviet Orders US Vice Consul Expelled as Spy": http://www.…
Unless I missed something, those articles just give specific examples of agents being uncovered. Nothing is given to indicate how many remained uncovered. Its a pretty long jump from examples of such to the general statement that "the majority of official-status officers" being known by their host countries.
>"United States counterintelligence specialists estimate that at least 150 K.G.B. officers are working out of the embassy under cover as diplomats, clerks, chauffeurs, journalists and in other jobs."
Just as a point of reference, the US Embassy in Iran during the 1979 revolution and subsequent hostage crisis had something on the order of 75-100 people working there.
Again, Google around, there is a ton of material out there on these topics. If you read Russian, I'm sure there are similar stories talking about US spies. (Although the reliability of Soviet media is questionable at best) I was really interested in this stuff when I was a kid, and recalled these events vaguely from those days, there's plenty more to find.