The writings and sayings of Karl Marx were frequently used in the Soviet Union around 1974. All Soviets in 1974 would have read some of his works or being taught about his theories at some point in their schooling (excusing those that may have attended school before 1918). However, the man that was most quoted and sourced in propaganda was Vladimir Lenin.

It also depends on what you mean by "read Marx". Yes, everyone had most certainly being instructed to read some of his writings. If you wanted to advance in the Party you had to read a lot of Marx. However, we can't say that everyone read the material, or even cared. There were certainly more people that had read Marx and thought very little about it than people that could be considered well versed in Marxist literature.

So, if you want a proportion of people that had probably read at least some Marxist literature (written by Marx), I'd put it at 90%. If you want the amount that were true readers of Marx, it would drop to a number that I can't estimate.