18 diapers should be plenty unless your ds is a very frequent wetter and you like to change him after every pee. We went through 6 diapers a day with one-year-olds. How do the size large fit? I found the large fuzzi bunz to be absolutely HUGE on my 18-month-olds. They didn't really fit until my boys were 2.5 (at which point, only one was wearing diapers and just for bed).