As indicated in previous responses, the correct answer to how many 6" blocks are needed to make a twin size quilt is--a LOT! There are too many variables in giving an absolute answer. Straight set or on point? Do you want to put these blocks only on the top of the mattress and border out the overhang? If you want to include them in the overhang, how deep is the overhang? Do you want to cover the pillows or not? But with only a 6'' block, it is going to take a lot of blocks whatever your intent.