Pretty sure Blizz said they are not going to allow mass/unlimited selection for competitive melee, though custom games probably will/should allow unlimited unit selection.
Personally I think the campaign should remain the same as competitive play otherwise some players may be confused/frustrated if they are different
A small increase from 12 to 16 or even 20 would be fine imo, because its not like you ever have 50 units unless your massing flying machines. I also think a “select all” feature would be bad for undead as it would select their ghouls and undead players usually leave ghouls on lumber.
There are better ways to improve unit selection without allowing unlimited selection, such as:
https://us.forums.blizzard.com/en/warcraft3/t/control-group-unit-removal-hotkey-in-reforged
https://us.forums.blizzard.com/en/warcraft3/t/pre-set-control-groups-in-option