This was first achieved by Julle Cesar so all credit to him for finding out how to do it.
To have more than 20 units in a stack: open 'saved game' in EsfEditor, CAMPAIGN_ENV / CAMPAIGN_MODEL, you will find 2 numbers with 20, upper is land and lower is ship'.
Just enter the number you need (don't know whether there are any restrictions, didn't test it) instead of 20.
It DOES work in campaign, and the limit is probably more than 80 units from each side (I am currently playing with stacks of 40, but have not reached this number yet).
Still in testing in my current british SP campaign to see what the ai does, might need to lower the unit costs a bit but will keep you posted with updates on my progress.
The only downside like in historical battles is that you can't see the extra unit cards for 21+ units so you might want to make a note of what units you have in a stack