All looms have a minimum amount of warp yarn that is required to set up (called warping) the loom. There is always some waste on each end.

To cover the cost of this waste, and to cover the labor to warp the loom ready to weave, there is a minimum amount of yardage where the weaver breaks even.

This minimum yardage amount is different for each mill. One mill I know of set their minimum at 36 yards while another is 11 meters. (There used to be a mill that would weave one kilt worth of single width fabric but it has been so long since I dealt with them that others will have to chime in if that is still the case.)

The mill I normally work with, for a custom run set their minimum at 11 meters of double-width fabric. This is one meter less than what would be needed to produce 3 kilts.

Of course the cost for a custom run will be higher, per meter, than a normal run where the waste and warping costs can be spread out over many hundreds of meters. (The per meter cost is almost double for a custom run.)

A custom run is the only option if you design your own Tartan of if the Tartan you want is one of the thousand of less common designs. This is where the help of a kilt maker can come in handy. One who deals with mills on a regular basis and is familiar with various mills. Their minimum yardage requirements, single with vs double width capabilities, and the qualities of the fabric of each.

I would be far less influenced by whether the kilt required a seam or not. Seams are the norm in kilt making. Single width is usually chosen because some Tartan designs are what are called "asymmetrical" which means that they do not lend themselves to seaming as well as symmetrical designs. The Canadian Maple Leaf, some Buchanan, Stewart Hunting, MacMillan old Sett, and Drummond of Strathallan are just some designs that are easier to work with if you have one full length of either single-width fabric, or the mill can cut a single length from a double width bolt.