12" truss ships 3 wide, 6 high for full carts. Partial carts sometimes ship 2 wide, but still no more than 6 high. This ensures that it fits through all standard doorways. We built our carts. 20.5" truss stacks 2 wide, 4 high. We use The Light Source aluminum dollies. All truss is stacked with the same orientation as it would be hung, with the ratchet strap laced in and out of the chords on the trusses. This keeps them from slipping around when pushed. All truss carts are loaded and unloaded by two people to keep people from getting hurt.
If I may argue this point for a moment. In my world, mass quantities of truss often arrive via a flat bed trailer, and are moved to point of use by forklift. Rolling each piece once, of the bottom layer only, onto its side, allows the forks to penetrate to better pick the stack. No dollies. I believe the standard for 20.5" is 3 wide x 4 high; for 12" 4 wide x 6 high. YMMV.
----
Material handling of truss is not a trivial matter. Some years ago, a colleague was killed during a load-out by a stack of truss falling off a forklift.
This is a very important point. We don't ship our gear that way due to differences in weather and climate. In the south, and especially the southwest, this is very common. I get twitchy every time I see forks going inside truss due to concerns about damage. Good hands and good fork drivers completely solve that problem. Also, I'd rather damage a piece of truss, rather than drop a stack of truss on a person.
How you load your truss at the shop needs to take into consideration how it will be offloaded and handled onsite.