Now, how much you can sustain at a maximum is a complicated question. Of course you have a hard limit. You can see the supply and fuel consumption of each unit in the Division composition screen, and thus, if the total consumption of all of your units exceeds your industry's capacity to produces supplies you're going to have a hard time. That said, it seems unlikely for a major power to even be able to reach that point. Unless your industry collapses due to resource shortages and/or Strategic bombing. To avoid the former (as Germany), you need ample stockpiles of rare materials and some big juicy Metal import deals with Sweden. (once you take DK, the Metal gets delivered overland, so no convoy issues.) Of course, supply production should be increased over time with the appropriate tech. The supply throughput, and supply transportation techs are also essential for big, far away, campaigns. Each unit of IC needs 2 Energy, 1 Metal, and 1/2 rare materials, per day to operate, so make sure you can keep your industry running for the duration of the conflict by stockpiling &/or not going overboard with IC builds. (unless you're playing the USA, in which case you have near unlimited resources.)
Second is your fuel production. For Germany, that's a problem, but in HOI3 you can mitigate it significantly by stockpiling Crude Oil, improving the Refining technology, and possibly the Coal to Oil conversion technology. (though that might be a problem if you've grown your industry so much you can't spare the energy). The larger your available IC, the higher the rate of oil being refined into fuel (as long as you still have crude to refine). Even with those measures in place, I would not advocate for building significantly more Panzer units than historical as those will use up your fuel at a ridiculous rate, especially once they start getting close to Moscow.