You really need to know how large the repeats are. You need to buy yardage in multiples of 6 repeats, so if they are 12 inches, a single set woudl be 2 yards. Assuming 3 3/4" high triangles, the pieced portion is only going to be about 24 by 36. A twin mattress is about 39" x 75", so if you want the whole thing covered, you would need at least 3 multiples of the 6 repeats, or 18 feet (6 yards)
The issue is that if you need to buy in multiples of 6 repeats, so if 2 X 6 repeats (4 yards) is not quite enough, you can't just buy another yard, you have to get 6 more repeats (2 more yards).
If the repeat is 24 inches, then you need to get 6 repeats, or 4 yards. If that's not enough, then the next multiple would be 8 yards.
You can go with the smaller number of repeat sets and then use borders to make the quilts the desired size.
Edited to add:
It is very important that the fabric is cut exactly square, so to allow for squaring it up when you get it home, add an additional 1/4 to 1/2 yard.