Less than one a year.
I don't think they collect specific "Mass shooting" statistics but this site compiles all deaths involving guns in japan GunPolicy.Org. As a commenter shared, the Wikipedia page has three entries Mass Shootings in Japan.
I think the political question underneath this is:
Why doesn't japan have US style mass shootings?
I believe that access to guns is surely one factor, but the biggest reason is their culture and society.
Japan has a culturally homogeneous society where children feel invested in their community and country. They have a culture of responsibility, respect, and traditional values. Such a society produces very few of the nihilistic, vengeful individuals who go on to commit mass shootings in the US.
In addition, a study on what US mass shooters had in common pointed to family violence, and a culture of idolizing other mass shooters as two major factors.
We can also point to countries such as Switzerland and the US itself before around the 1960s. Those countries had extremely high gun ownership but also had a homogeneous society and traditional culture. Nihilistic mass shooting events like we see in the modern US were practically unheard of despite the wide availability of guns.
With a similar number of guns available US mass shootings have been rising dramatically. What has changed is the US's culture, it's transformed from a Christian society with enlightenment values and hope for the future, into a hodgepodge of different cultures, peoples, and values. Many modern Americans feel increasingly disconnected from the society that they live in, disconnected from any spirituality, often without an intact family or any positive male role model. It's no wonder that so many become nihilistic, bitter, and vengeful and then act that out with a murderous attack.