In 1950, approximately how many Americans were members of the Communist party (including those who were undercover FBI agents)?
Wikipedia reports that after World War 2 the Communist Party USA had grow to about 50,000 by 1957 after a decade of losses had dropped to about 10,000. About 15% were FBI informants.
You would have to dig a bit deeper to find out the actual numbers in 1950. Obviously the Party was heavily compromised and not part of main stream America by the late 1950s.