Honestly, I don't think it's really too important at this stage exactly how many copies Shenmue III sold. It probably wasn't great. The sales probably never will be especially great, in terms of numbers moved, but none of the parties involved have called it a disaster like Marvel Avengers, or anything. So, I don't think it's particularly worthwhile to try to play these numbers games with it.
I think the surest sign that we shouldn't be that concerned about the sales for Shenmue III is how little Yu Suzuki is apparently involved with the animated series. That suggests to me that he's spending most of his time working toward the next game in the series, and he likely wouldn't be doing that if Shenmue III had been a financial liability.
Of course, I'm sure driving more sales of Shenmue III couldn't hurt, but trying to yard-stick it now is probably not what fans of the series should be focusing on, I feel.