The limit per province is ~800 sure. Every province around capital can pump out max supplies, so theoretical limit towards the front is in thousands! It can get bottle-necked somewhere, as per prov capacity is halved in Poland/SU, but still, there is no technical limit as long as your supply is not going through a single path. I've seen quite a few paths, sure they have minds of their own, but each can pass supplies.
So that's where infra building is golden, I start it immediately I occupy Poland in 1939, and by the time of declaration I have almost perfect infra ready ( and practical hot for upcoming SU campaign).
Here lies the flaw of your thinking - waiting for something, and why for 10? maybe 5.15 is already enough, and 6.45 will handle future needs when battle for Stalingrad will start? The thing is, paths are multiple, supplies flow in rivers, so upgrading infra can have positive results and very fast. Consider this: if you upgraded those paths by 1 level and started at levels 4-6, your total network capacity jumped by 50%. All that in ~4 months, and effortlessly if you are prepared to build infra.