Soviet Union football league system changed many times following the introduction of national league competitions in 1936.
Soviet Union football league system Wikipedia
Initially, for the Spring championship of 1936 there were only seven teams participating in the top group, while for the Fall championship amount of participants was increased to eight. A similar case scenario took place in the second group as for the spring championship saw some withdrawals. In the third group the number of participants was unchanged at eight for both parts of 1936 season. At same time in the fourth group initially there were five teams for the spring half and six for the autumn.
In 1937 the number of participants continue to grow. Some reshuffling took place between groups A and B, although initially both consisted of eight teams. Groups V and G increased their team pools to 10 and 12 participants, respectively. Also for the 1937 there was introduced the Group D with two subgroups.
In 1938 all of groups were merged into one big super league of 26 teams.
In 1939 and 1940 season Group A consisted of 14 teams. There also was a revival of Group B with 23 teams at first and then reduced to 14 as well.
In 1941 season there was only Group A championship which was heavily adjusted to 15 teams.