I'm going to add in another variety of greenery to give a nice mix of texture and color. You can see a single bunch of myrtle is pretty big with lots of branches.
Being forced to buy 10 bunches at a time seems rather excessive to me. Plan your flowers and greens out ahead of time and buy only what you need.
You can see what nice, long laterals some of the branches can be. This is going to add a lot of height and width to your floral arrangement.
Some are so long that you may have to cut them down into shorter pieces for the center and front parts of the design.
I put the tallest piece in the back and angled slightly backwards. I then begin to shorten the next pieces, angling them out to the sides as shown.
Finally in the center use the shortest pieces, inserting right into the center of the design.