You will get the 21 credits. Since you don't get credits for your first day of arrival, you will use at least one/two there depending on where you start your dining journey.
My 21 credits looked something like this:
Arrival Night - California Grill (2) - Saw Wishes Fireworks - beautiful
1st Day - Akershus Princess Breakfast at 8am. (1) Easy FP for Soarin. Garden Grill Dinner first possible reservation (1)
2nd Day - Tusker House (1) at 8am. Easy FP for EE, 1st people on Safari, Boma (1) 5:00, left just after the parade
3rd Day - CRT (2) 8am. Beautiful empty park, no lines for Space Mountain. Dinner at Ohana (1) 7pm.
4th Day - H&V Breakfast 8am (1) Better than sitting in rope drop line for an hour. Mama Melrose Fantasmic Package (1) 5ish
5th Day - Crystal Palace 8am (1) no lines for Splash or Thunder. Hoop De Doo Musical Review (2) - DS9 laughed so hard he fell off his chair literally
6th Day - Went to Epcot for EMH, left for late breakfast at Cape May Cafe (1), Le Cellier Lunch latest possible reservation (1)
7th Day - This was leaving day for us. Breakfast at Ohana's 8am (1) Finished check out. DTD for last minute shopping, late lunch Raglan Road (1)
This left us with a few extras and we used them at QS. We usually split a meal for lunch if we were hungry. We did Columbia Harbour House once, Flame Tree another. By sharing them, we didn't end of paying OOP out of any meals.