And statements from producers working at the labels themselves.
The producer for
Scitron Digital Contents, Takahiro "T.J!" Yagi, had told me that the average for albums from Scitron was around 4~5k. For an average, that would mean some would do more, but some would do less. (That conversation was back around 2002).
Of course, the year of release matters a lot in this context. An album from 1987 would have sold more copies than an album from 2007.