There were, by very rough estimates, around 24 million WW2 veterans from the USSR. The reason I say "very rough estimates" is because as a result of the Stalinist purges the military bureaucracy was in disarray when the war broke out in 1941. This, coupled with the chaotic situation in, primarily, the first year or two of the war lead to Russian casualty figures being notoriously unreliable. Another flaw of the Russian reporting system is that there were, at least initially, no unified reporting of casualties, soldiers killed in action were reported by their respective commanders but the figures for rear area casualties (soldiers wounded in action and evacuated) are even more unreliable and chaotic.

We know that russia conscripted some 34 million men and about half a million women over the course of the war, this figure at least is reasonably reliable. As for casualties the figures range from around 8,7 million dead (from the report to the 1993 report to the Russian Ministry of Defense by Colonel General Grigoriy Fedotovich Krivosheyev (or Krivosheev, westernizing Russian is trick, his name is Григорий Федотович Кривошеев). However, Krivosheyev does not include casualties among rear area civilian support personel, conscripts killed before they could be listed as active duty, partisans etc. Basically, Krivosheyev lists only the dead among active duty Red Army and Navy. Krivosheyev also puts the numer of wounded at around 14,5 million and the number of sick at 7,5 million. He does however include those who died in German or Japanese captivity in his figures (provided they were listed as active duty at the time, again, no partisans, civilian support personel etc) which were about 1,2 million of the total 8,7. The Ministry of Defense has since updated its figures to 8,8 million although I can no longer find the official statement for that. It should be on the Ministry of Defense homepage, the report was released in mid 2016).

That said, there has been valid questions surrounding Krivosheyevs figures, perhaps most notably because a very significant portion of the casualties from 1941 and 1942 were only estimations. This is because a significant portion of them comes from units that were encircled, surrendered or overran in the early phase of the war. These units generally did not produce accurate casualty reports and if they did, were unable to relay them anywhere. Consequently, Kirovsheyevs figures are estimations or extrapolations based on incomplete data.

Other critics of Kirovsheyev suggest that the data for soldiers reported missing in action is understated as well as casualties in rear areas. Kirovsheyev uses a figure of about 18 million wounded or sick whereas the official figures from the USSR medical services put the combined total at over 22 million. According to Kirovsheyev the discrepancy is explained by the fact that official sources from the USSR medical services also include wounded and sick civilian support staff, partisans and other non-active service personnel.

Sergey Nikolaevich Mikhalev (Сергей Николаевич Михалёв) published a study in 2000 which puts the number of casualties at almost 11 million. He claims that Kirovsheyev underestimates both the number of POWs and the number that died in rear areas as well as those executed for various offenses. He does however not dispute Kirovsheyevs overall figures for KIAs among active duty Red Army and Navy personnel.

There are other critics of Kirovsheyevs figures who have point out that there are other possible sources of unreliability that include, among other things, unclaimed bank deposits, discrepancies in front and rear hospital figures, understatements of pre-war populations, unexplained demographic changes etc, however, for the purpose of this post I will use a rough estimate based on Kirovsheyevs and Mikhalevs figures and put the number of military dead at an estimated 10 million.

If we assume that these figures are correct that leaves a total of, roughly, 24 million WW2 veterans from all notions within the USSR. This figure is higher by far than that of any other nation by similar calculations. I'll include some figures here for reference (Total served - total KIA, not including civilian casualties):

Germany - 13M

USA: 15,5M

UK and dependencies - 5,5M

Japan - 6M

However, your question is why these people aren't on the Wikipedia list... I don't know what to say. Wikipedia does not maintain a comprehensive source of WW2 veterans, it contains only notable people as arbitrarily decided by whoever created the respective persons wikipedia page. Since it is a primarily anglophone resource it has a natural bias towards western and anglophone sources which leads to an over-representation of "notable people" from anglophone countries.

TL;DR - There were a lot of USSR veterans, more by far than from any other country, they just don't have wikipedia pages dedicated to them.

I would like to include a small additions to the great response above. First, the country that took part in the Second World Was was the USSR that does not exists anymore after it dissolved more than 25 years ago. This means that former Soviet veterans now reside in Russia, Belarus, Ukraine and other autonomous countries that became a home for almost a half of the population of the USSR (the population of USSR in 1990 peaked at 291 million and the population of Russia in 1992 was 147 million). This means that half of the potential Soviet veterans will not be included in Russian statistics.

Now, due to various factors, including pollution and economic development, average life expectancy in Russia is significantly lower than that of many Western countries. According to WHO surveys for the year 2015, average male life expectancy in Russia was 64.5 years (with 66.3 in Ukraine, 66.5 in Belarus and 65.7 in Kazakhstan), far below that of Germany (78.8), United States (76.9) United Kingdom and France (79.4), United States (76.9), Japan (80.5) or China (74.6). It is even low in comparison to former Warsaw Pact countries such as Poland (73.6), Hungary (72.3) or Romania (71.4). This means that a lot of Soviet veterans simply died long before their contemporaries in other countries. There was also non-negligible increase of the death rate after the dissolution of the USSR that, along the low birth rate and emigration resulted in significant decrease of Russian population in the last 25 years.

Of course, one should note that any WW2 veterans who have been an adult during the war must be older than the listed average limit, given they need to be at least 92 in 2019. Nevertheless, there is still more than a bunch of veterans. According to the official data of the Ministry of Labour of the Russian Federation (that, unfortunately merges the number of war veterans and war invalids), as of May 2019 there are around 75.500 veterans of the Second World War still alive in Russia and further 36.000 living in the countries of the former USSR (Ukraine, Belarus, Kazakhstan, Uzbekistan, Kirgistan, Turkmenistan, Tajikistan, Lithuania, Latvia, Estonia, Moldavia, Georgia, Azerbaijan and Armenia were taken into account).