Are you planning to use strips between your columns of coins? A border? Those factors will determine how many charm packs you need, for sure. There's also a lot of variation in what people think of as twin size -- some like a lot of overhang and enough length to fold over a pillow, and other approach the size more as just covering the top with a few inches over -- all a matter of preference. I'd suggest that you figure out how big you want the end quilt to be, then draw a little sketch including how wide your sashing columns are and any borders. I use regular old graph paper to help me draw a plan.
RSt