Well, a human japan should be able to mobilize all its manpower into the miltary relative soon, before 1942 for certain. But similar to germany japan starts a with a lot of manpower relative to to its daily gain of manpower. Thus you may have a big military that in battle and due to attrition looses quite some manpower each day. So running into manpower issues is likely unless you properly prepare for it.
Donnot mobilize all manpower. Keep 20 to 50% of your current manpower in military as unused manpower. So if the mouseover on the manpower symbol on the top or in the production menu says you have 2000 manpower in the military, than you should keep 400 to 1000 manpower in the pool. Keeping proper manpower reserves is important for most major nations, namely germany, soviet union, uk, japan, china, italy and maybe france. The USA are a bit of an exception, as they start with very little manpower, thus shifting the focus in favour of having a greater portion of available manpower mobilized.
As japan a key element is to manage your puppets. Manchukuo, a puppeted China and a puppeted India can give you a total of 250 divisions, thus effectively doubling your army size. This might allow you to outnumber soviet union. Techrush infantry divisions for high fighting efficiency of both japanese divisions and those of your puppets.
Always keep in mind attrition. The low ESE resulting from low infra and partisans and the climates japan encounters in the far east cause a great deal of attrition. Logistic wizards(-50% on attrition except hostile front attrition) and possibly the -15% supply consumption ministers may help with that. But optimizing for a proper supply chain(meaning routes that "carry" ESE) and minimizing exposure to bad climate and hostile front attrition is likely the major consideration on the attrition front. Without proper consideration for attrition, attrition may cause bigger manpower losses than actual fighting. Chances are that with proper attention on both attrition and fighting, attrition losses will still outweight combat losses in say a 5 year average.
