What the title says. Germany should always get out 10 units a turn, because between having to invade one major power and fend off two others from behind, they are going to constantly suffer attrition, and while getting more expensive units might allow for greater offensive pushes, it won’t matter if all of the sudden you run out of units.
I think this is true even on turn 1. Germany should get 10 Infantry on G1, because they have no real chances of taking Moscow on G3 or G4 anyways, and those 10 Infantry will be able to reach Moscow themselves by G5. Unless Axis seriously intends to capture Caucasus on I2/G3 and hold it till I3/G4, a 6 Tank build is going to be of limited use. However, the earlier that Infantry are purchased, the more likely you are to get full use out of them.
As for G2 and beyond, you have several options, assuming that you have around 52 Income (Karelia/Baltic States/East Poland/Ukraine), for each 2 income less than that you can build an Infantry instead of a Tank:
10 Tanks: If you get this G2 and G3, you can hit Moscow on G5 with your builds from G1 through 3, for a net of 20 Tanks and 10 Infantry, excluding what you started with. This should probably be enough to kill Russia, but it doesn’t leave much for Defense.
6 Infantry, 2 Tanks, 2 Bombers (Good SBR and defense)
5 Infantry, 3 Tanks, 1 Fighter, 1 Bomber (Balanced)
4 Infantry, 4 Tanks, 2 Fighters (Efficient offense)
7 Infantry, 3 Fighters (slow but very efficient both on offense and defense)