Sorry, I misremembered the exact numbers.
https://fat-yankey.livejournal.com/32078.html
Admittedly not on par with an academic source, but based on this post, all acquired motor vehicles in the USSR (captures, domestic production, and lend-lease) throughout the entire war totaled to about 745k vehicles (all vehicles, not just trucks). Of those, 352k were lost. The post seems to be based on some Soviet memorandums, but I cannot verify them (due to not speaking Russian and not having access to the memorandums in question. Some figures are also corroborated by other online sources too.
As for why, the Soviet Railways made up that difference. I mentioned in an earlier post that Germany neglected their railways from 1933-1939, which had ramifications for what rolling stock they had available by Barbarossa. The Soviets had something like 5-6 times as many locomotives as the Germans on the Eastern Front for most of the war, and they also had larger and better organized logistics and engineering troops that could regauge railways, repair infrastructure, and load/unload equipment closer to the last mile, thus making up for having less motorized vehicles.
Edit: This thread describes a lot of the issues the Germans had with the Soviet railway network:
https://forum.axishistory.com/viewtopic.php?f=66&t=203286
Basically, the Soviets had a low-speed, large train, long-distance rail network, whereas the Germans (and the rest of Europe) had a high-speed, small train, short-distance rail network. The size of the Eastern front and the sheer scale of war logistics meant that the German network was unsuited to logistics it need to sustain long-distance pushes, but the Soviet network was perfectly suited to do so. In addition, the larger prewar Soviet railway network meant that it had a larger base of skilled laborers from which to expand its railway corps.