SC2Mapster

Under the Requirements data type create a new requirement. Double click the Basic: Requirement - Node Link field to open a window. There will be two folders with one called Use and the other called Show. Right click the Use folder and select Add Requirement Node. Then left click your new node and under the Type field select Less Than. In the window right click your node and add select Add Requirement Node twice. Set the upper one to the Count Unit type and the lower one to the Constant type. Under the Count Unit node set the Alias field to the unit you want to limit the number of and the State field to Queued Or Better. Next set the Constant node to the number you want. Finally add the requirement to your Build ability.

Add Requirements to the Build ability used. Under the Use section of the requirement have a Less Than node that counts the numbers of the unit that are completed.