NTT Docomo Factbook lists the number of subscribers for each of the Japanese MNO as well as their market share. At the end of 2018, according to Telecommunications Carrier Association (TCA), the following was the market share:
NTT Docomo - 77.1 million subscribers (45.1% market share)
KDDI - 53.5 million subscribers (31.3% market share)
Softbank - 40.4 million subscribers (23.6% market share)
NTT Docomo provides a complete breakdown of it's mobile subscribers which is available here. The forecast is to have 78.1 million subscribers by Q3 2019.
KDDI au subscriptions info is available here. At the end of 2018, there were 54,262,800 subscribers
Softbank on the other hand, has 33 million mobile subscribers and 7 million broadband subscribers according to their web page. Their broadband service contains of Softbank Air which is high speed 4G wireless internet as well as Optic Fiber service.