... any guess as to how many STZ-3 tractors the Germans were able to impress into service in the summer of 1941? Some would have been from agricultural collectives that were overrun.
It was a windfall, no doubt ... but slow
The Russian references don't state the number of the captured STZ-3 tractors. It is mentioned only that the great number of the tractors was caught. Judging by the total number of 16.000 vehicles assembled in 1937-1941 , it should be quite a lot. The Red Army was delivered with 4000 of the STZ-3 tractors in the same time. I mean the period of 1937-1941.
... do you think many of them survived Operation Typhoon at Moscow, December, 1941?
I cite that date because that's when the 'panjay' horses came to the rescue of the Heer
Contrary to Soviets , Germans didn't consider the STZ-3 tractors to be a great artillery tugs because of its small weight and speed. So these were moved to the auxiliary "lighter" works. Especially these were useful at airfields towing heavy bombs, aircraft or rollers. Also these were used as tugs on the muddy or snowy roads. Because of that many of them could survive. But later without the spare parts, the number of them could be reduced considerably.
Here is a nice site with many interesting shots...
IMHO the tractor was not enough to town a such heavy tank. If you have a look at the enlarged part of the shot you will notice a steel line there. I think there was one more or two tugs. Also it is possible that the KV tank with the tractor together could be trying to pull something.