How many German soldiers fought in the invasion of Russia?
I've never seen a detailed report on this matter. However a few conclusions can be made.
Germany started the invasion in June 1941 with about 2.2 million men. Casaulties totalled about 1.0 million from all causes by the end of that first year. Replacements were not 100%, so perhaps .5 million were added. That brings the total to 2.7 million men engaged during 1941.
For 1942 a similar number of Germans were engaged plus about .5 million support troops from Romania, Italy, Spain, Hungary and Slovakia. Since most of the Germans were veterans of the '41 campaign we should only add another 1.0 million or so for 1942.
During all this time the Germans were also rotating troops in and out to other fronts, on leave, etc. I suppose you could add another 20% or so to these numbers so that is taken into consideration. Thus you'll be up to 4.3 million individual men engaged at some point in the Russian campaign.
For 1943 I suspect we should estimate a number similar to that of 1942, ie, another 1.0 million new recruits plus the existing veterans.
By 1944 the front was mostly back to the Polish border or Germany proper. Here the number of Germans engaged become very problematic.
Thus I would estimate, very roughly, that between June 1941 and late 1944, about 6.0 million German soldiers were engaged in the fight with the Soviets. German peak strength never topped 3.0 million at any given time, although total numbers at the front sometimes exceeded this due to allied armies.