There is no definite answer to this because many publishers are involved, and often they never reported sales (because they were not paying the royalties they owed Heinlein!). However, the total answer cannot be less than many tens of millions. Stranger In a Strange Land by itself has sold more than (conservatively) 25 million copies (and possibly many millions more). An estimate of 80 million copies altogether appeared on the back of a new issue several years ago.