Here's a couple more shots of the mass production of trench sections.
You'll see that in some sections I've cut an access trench. This can be used to link first and second lines with straight trench sections (as communication trenches) or to allow a Sap to run out onto no-mans land. They also allow bunkers to be joined to trench systems.
In all you're recommended to have 36 sections for the basic game, 18/side so I'd better get a move on.