First off, you will need a good text editor with line number display. I would recommend a program like Notepad++.
Go into your Zero Hour Directory which is usually:
C:/Program Files/EA Games/Command & Conquer Generals Zero Hour/
Locate the "Data" Folder, then the "INI" Folder, then the "Object" Folder.
Open the File with your text editor entitled:
Go to line 4875. It should say this:
Slots = 10 ;Note : to lucas, change this to 50 for your fun tank!
All you need to do is change that number to anything you please. =-D
Save the file and have fun!!!
To check it the quickest, I would recommend 1v1ing a hard army and using the Domination Army. Just get to rank 5 and use the Stolen NEO Generals Power