According to Famitsu, in August, Japanese gamers bought 83 thousand cartridges with Nintendo Switch Sports.
In fairness, we note that in this chart, versions of games for different platforms are considered to be separate titles (i.e., a game released on the Nintendo Switch, PS5 and Xbox Series X can enter the chart three times). Therefore, if you look at the total sales, then the best—selling game in August in Japanese retail will not be Nintendo Switch Sports, but Earth Defense Force 6 – it was bought 92.6 thousand times.