What are these white numbers at grandmaster icons in char select? I belive they should be connected with their place on ladder, but what exactly are they? Their place? Their place among the same hero players? Something else?
These numbers refer to the position that player is in the Top 500 in their respective region
This number is irrespective of their hero of choice, and is region-wide. In addition you can check this in the leaderboards when right clicking the competitive search button.
In the leaderboard view you can check out the 3 most played heroes for each player in the top 500 for each region, as well as their SR.