Soviet WW2 veterans seem to have an amazing number of medals on their uniforms. What accounts for such a large number of medals?

They are two main reasons for that.

First is that most of whose medals are commemorative or generally not -war related, like 20 Years of Victory in the Great Patriotic War 1941-1945 medal or 60 Years of the Armed Forces of the USSR medal- those there given not only combat veterans, but also to everyone who served 20+ years in military or already were awarded for personal achievements while in service. In short commemorative medals were given in millions.

Second, veterans likes to show up with all the medals and badges they accumulated in lifetime, and in USSR there was a lot of whose. There was medal given on various jubilee medals (like In Commemoration of the 800th Anniversary of Moscow), medals for good work (For Distinguished Labour) medals for serving in various institutions for 10, 15, 20 years, there was even medal for surviving until retirement and so on. So elderly peoples in former USSR often have many medals and badges, not to speak that some people just buy them now and during such celebrations you will often see a "veteran" with highly improbable or outright impossible assembly of awards.

So in short, your typical veteran will show up on 9 of May with something like this

1- Commemorative "100th Anniversary Since Lenin's Birth" medal (1969)

2- Commemorative "300 Years to the Russian Fleet" medal (1996)

3- Commemorative "20 Years of Victory in the Great Patriotic War" medal (1965)

4- Commemorative "50 Years of Victory in the Great Patriotic War" medal issued by Communist Party of Russia (1995)

5-6 unidentified

7- Commemorative "90 years to Soviet Armed Forces" medal issued by Communist Party of Russia (1997)

8- Commemorative "Veteran of Soviet Armed Forces" medal (1976)

9-11- "For distinguished service" medals awarded to servicemen of the MVD and KGB (circa 1958)

12- unidentified

14- Commemorative "50 years to Soviet Armed Forces" medal (1967)

15-18- unidentified

19- Commemorative "Veteran of the Navy" medal (modern)

20- Commemorative "Medal of Zhukov" (1994)

21- Commemorative "50 Years of Victory in the Great Patriotic War" (1994) (there is a typo in picture)

22- modern award with Russian coats-of-arms

23- unidentified

24- "Veteran of naval war" medal (modern).