To set an average attendence for a league or a tournament, you must go to Fifa16/data/fifarna/lua/assignements and open the general.lua file
and then write or modify lines like in these exemples :
setTournamentCrowdSize(232,6,2) --Uefa supercup
setTournamentCrowdSize(16,5,2) --France Ligue 1 (1)
where first number is the tournament/league asset id, second the home fans and last away fans (here 232 is uefa supercup, 6 for full capacity, 2 full away block).
As Nikolapfc said home fans values go from 0 to 6 and away values from 0 to 2.
You can do this with any league or tournament, i even do it with pre season tournaments, all you have to know is the asset id.
You can also set the attendence for your particular team vs another team. For exemple if you have a big derby you can set it to (X,6,2) for a full stadium and for a game vs an average or weak team you can write another line with for ex ((X, 4,1) for a less full stadium.
For this you must go to Fifa16/data/fifarna/lua/assignements/teams and there open or create the team_X.lua file for your team where X is the id of your team. Then in that file you can again write a line like this:
setTournamentGameCrowdSize(71,73,16,6,2)
here i set FC Nantes (71) when playing vs PSG (73) in french ligue 1 (16) home crowd must be full 6 and away block also (2)
You can do this with any team vs any team of a league and even in cup games, i find this very cool and its a shame EA doesn't use it.
Here is my team.lua file for Nantes so you can see there is a lot of possibilites:
http://www56.zippyshare.com/v/J7xDxRmA/file.html
But as i said, the repartition of the crowd in the stands doesn't look as realistic as the one you did edit in blender.