Pavement Stencils - 1/16" (63 mil) or 1/8" (128 mil)?
I'd suggest asking your painter. They might have a preference, and you might save money overall by following their preference, if they have one. Cheap materials that waste paid professionals time can be VERY expensive...or they might already own a set of number stencils, or know where to borrow/rent a set inexpensively.
Depending on materials, the thicker ones might stay put better and save you money overall if the painter does not have to spend lots of time fussing with a thin stencil that does not want to stay put, blows in the wind, etc.
This is more likely if it's a flexible/rubbery/plastic material.
Then again, it might just be a durability issue (how many uses you'd expect to get from them) and make no difference for limited use.
This is more likely for a stiff/rigid stencil material.