Not sure you are talking about Master Documents or simply Sections. If you are talking about Master Documents, rethink it. Seriously. "there are two kinds of Master Documents: Those that are corrupt and those that will be corrupt soon."

Word can handle just about any size document with today's computers.

You can copy and paste or if you want to continue to have separate files for parts use IncludeText fields.

There are a few valid reasons to use the Master Documents trap in Word but doing so must be done very carefully. People have lost years of work using this trap, don't become one of them.

If you are talking about Word Sections, they are numbered sequentially. That does not mean that you can't have a heading numbered or labeled S2Extra. Sections / Headers and Footers in Microsoft Word 2007-2013