At long last, I have finally found an interesting document on German airfields in Russia: https://www.ww2.dk/Airfields%20-%20Russia%20and%20Ukraine.pdf
That document lists 1917 airfields that were or could have been used by the Germans in European Russia during the time and in the territories they occupied. But it must be noted that the Soviets themselves built many thousand more during their own advance in the later stages of the war, and those that the Germans abandonned were usually so thoroughly destroyed that the Soviets could not use them again. But the German advances were usually so fast that they could lay hands on usable abandonned Soviet airfields after some light repairs.
Here is the introduction, which explains both the extensive use of existing Soviet airfields and the capacity to create new ones on the fly:
The story of German-built airfields in Russia is really the story of Russian built
airfields in western Russia, at least for the most part. As the German
forces advanced into Russia, they captured, repaired, refurbished and
improved existing Russian airfields the Luftwaffe believed to be usable and
also used undeveloped grass fields which required very little construction
work for their light weight, single-engine aircraft. These grass airstrips
were used for a few days or weeks and then abandoned as the front moved
forward. The main exception to this rule would be forward airstrips built
just before a German ground offensive was scheduled to begin, such as in
eastern Poland in early 1941 prior to the 22 June 1941 invasion of the USSR.
Large numbers of Luftwaffe construction troops were ordered to Poland and
more than 100 airfields, field airstrips, landing grounds, and satellite strips
were hastily built in the eastern part of the country. Other notable
exceptions were in May-June 1942 for the German advance toward
Stalingrad on the Volga, in the Orel-Kharkov area in spring 1943 in
preparation for the Kursk offensive that began on 5 July of that year and
finally, to a lesser extent, in late 1943/early 1944 in Ukraine to
accommodate Luftwaffe units withdrawing west of the Dnieper River and
transport units operating cargo flights from Ukraine into Crimea.
Otherwise, probably 85% of the German airfield construction work in the
East was to repair, refurbish and improve existing Russian airfields and
airstrips as needed.
"The closer you get to the meaning, the sooner you'll know that you're dreamin'" -Dio