Whelping is the one use where a pad is changed several times a day, and where running out of clean ones is an actual problem rather than an inconvenience.
Take the largest size
36 x 41 in (91 x 104 cm) is the size built for this. A whelping box needs a surface a mother can lie along fully, not a target she has to aim at, and puppies don't stay where you put them.
Washing frequency is the real constraint
Independent care guides for the category recommend washing every two to three uses. During whelping you'll be well past that, so the limit on how many pads you need is how fast they come out of the wash, not how much each one holds.
Air drying takes hours rather than minutes, because it's a genuinely absorbent pad. Tumble dry low, below 40C, since high heat is what wears out the waterproof backing. That's why these ship as a 2-pack, and why more than one 2-pack is sensible for a litter.
What to avoid
Bleach and fabric softener both attack the layers you're relying on. Softener coats the top layer so it stops absorbing, and heat and bleach break down the waterproof backing. The anti-slip base is the part that goes first, and once it splits the pad is finished.
Full washing detail is in the care guide. Sizes are on the washable pee pads page.