The notation AA refers to the size of the bloom that will come from the plant (AA is the largest Dahlia blooms) - the size of the tuber doesn't have anything to do with the size of the blooms.
A #1 sized clump? must be a Dutch thing, as they are the ones usually selling clumps. Most American dahlia dealers sell individual tubers. (a clump refers to a group of tubers all still connected to the stem they grew off of) #1 must be the largest size clump they sell. You should be able to divide it and grow multiple plants.
