Just do 30 units a stack, catapults and other huge things cost 3 spaces. Then make it modular like the chain of command is currently. More than 20 units incurs a -15% defence penalty due to clumps of people being easier to hit than one guy.
That way the problem is solved and people who prefer it with no limit can just remove it.
End result: Everyone's happy. Which is kinda the point
(Numbers made up, but thats how I'd go about it in any case. Catapults costing triple is so people have to actually defend them from all sides instead of just sticking units on top of them. Makes weak points in defence more likely, and stealth units more useful.)
Also, talking about realism with supply chains is kinda silly when "screw it, throw some fireballs at 'em" is a perfectly logical thing to say.
That way the problem is solved and people who prefer it with no limit can just remove it.
End result: Everyone's happy. Which is kinda the point

(Numbers made up, but thats how I'd go about it in any case. Catapults costing triple is so people have to actually defend them from all sides instead of just sticking units on top of them. Makes weak points in defence more likely, and stealth units more useful.)
Also, talking about realism with supply chains is kinda silly when "screw it, throw some fireballs at 'em" is a perfectly logical thing to say.