It might have been better to ask this question in a separate thread, for me at least all questions on markings are important.
Talking about the Russian trucks from the 1941-45 period, the numbers were big on the tailgates only (and respectively on the rear of other bodies). This was a rule for all lorries over 1 ton load capacity, civilian and military either.
Talking about the military trucks (from the same period again), no number plates on them in that country, in usual meaning of separate pieces of sheet metal fitted to the front and rear. The vehicle number was painted (often freehand) on the doors, both sides, and was repeated on the rear with big letters/numbers which may considerably vary in size and shape (freehand, stenciled, etc.)
The military number system for the softskins over 1 ton capacity was a single letter followed by a single number, and two group of numbers, two each, all separated by dashes. Something like that: A-X-XX-XX (where X is a number). The unit was coded in the first part of the number (A-X) and the two groups after it (XX-XX) were in fact the vehicle number within the unit. Sometimes the number was preceded by a small white star, like this: *A-X-XX-XX. I was told this was a sign for Guard units. The Polish units in the Red Army used small white Polish eagle instead, etc.
It seems there were no standards for those numbers, except for the placement.
Also, on many truck rears big squares or circles were painted on the left for blackout marking. Often specific unit emblems and/or markings were also present.