>>Would these work for 15mm scifi ?
Yes. I have a few 15mm Sci-Fi-Fi tanks, and all the markings would look in scale on them as turret numbers, logos, etc.
>>How many sheets per set ?
1 sheet as you see. Though I can rescale and resize any set.
>>What would it cost to print one's own art ?
It depends on the size of the sheet you have, and how many sets you want. For color fidelity, the artwork needs to be a vector file, or converted into one. A custom decal set can be anywhere from $10 to $70 depending on the size.