After a J1 attack, and if Japan take planes as casualties, they will have 3 inf and 1 art in Yunnan, and that is hard to trade when China attack with 4 inf and a fighter. So basically the best strategy is to just forgett Yunnan and walk a giant stack up north to threaten the money territories at the coast after Japan have taken India. That is if you want to win.
5 inf, if you include the one in Kweichow