Hi, Laura! Your number of nights of your Disney Dining Plan will be the same as the number of nights you are staying at your resort. So if you are planning on a 6 night stay, for EACH night of your stay you will get 1 Table Service Meal, 1 Quick Service Meal, and 1 Snack per person. Click
HERE for more details on dining. You cannot just purchase for 3 of your 6 nights.
You can, however, have a different number of days of tickets on your package (must have a minimum of a 1 day ticket on your package.) Dining only corresponds to the number of nights you are reserving.
If this seems like it will be too much food for you, you can always choose to just pay as you go, and not do the Dining Plan. Either way, make sure to call for your Advance Dining Reservations at (407)WDW-DINE.