Broadly speaking: They usually do around the 7-8 million mark, which is about half of what the initial pair does.I should add that I don't doubt that there are people who will wait for "Stars", but yeah... When that leak first came out I saw quite a few people who legitimately thought it would seriously hurt sales of Sun/Moon.
I really wonder what this means for the Switch version. I'll have to go look up some data on how other "third version" titles did.
This thread has detailed sales data.