Age of the Empires ll: Age of the Kings

I don't think it's in the map editor.

This is what I found, dunno if it works.

In Windows 95, click the Start button, point to Settings, click Taskbar, click Start Menu Programs, click Advanced, and then double-click Programs. -or- In Windows NT, right-click the Start button, select Open All Users, and then double-click Programs.
Double-click Microsoft Games, and then double-click Age of Empires.
Right-click the Age of Empires shortcut, click Properties, and then click Shortcut.
In the Target text box, type Limit=x after the existing command line text. Type the population limit instead of the letter "x." For example, to set the population limit to 65, type Limit=65. You can type any number from 25 to 200. The default setting is 50. Parameters must be separated by a space; capitalization is not important.

how do you do that for windows xp

Use the Windows NT instructions.

Is there a way inside the game to change the population limit. I tried your method and it didn't work.

I don't know but I do not think so. Increasing the population can crash your computer so they make it intentionally in a way that only techies know how to do it.

I've never played the game so I fear you've seen the extent to which I can try to help already.