How to limit the maximum no. of units owned by XXX ?

Besides using the train trigger or limiting the food ?

What is wrong with the food cap? When or how do you want it stopped. Don't be too general.

Does reducing food cap work for limiting neutral hostile units ?

If so, I want to know are there other ways to do so.

Event: A unit enters (Playable map area)
Condition: Player owner of (Triggering unit) equal to (Player X)
Condition: Count units in (Units owner by (Owner of (Triggering unit))) greater than or equal to X
Action: Unit - Remove (Triggering unit) from the game

There are some problems with this, such as not giving the gold back...

You can use any actions besides from remove, such as making the unit type of the triggering unit "untrainable"

you do it by
event - unit starts training unit

action - if(number of units owned by player xxx is less or equal to xxx do nothing els cancel training unit