Max number of builds?

Hi, we have a Windows 7 .WIM that is about 4GB in size.   We have a workbench area that can build 16 machines at a time.  We've noticed that the max number of machines we can build at once is 12.  As soon as we try for more machines will fail to complete or they complete but are missing software or software will not launch.

When we build 8-10 machines at a time they all work every time.     Is this a concurrent connection issue or a data throughput issue?  

Thanks.  

mqh7