Depth is where most people go wrong
Mulch should be 2–4 inches deep: two inches is enough to hold moisture and block weeds in a well-established bed, three to four inches suits new beds and harsh summers. Past four inches you start suffocating roots and inviting rodents — more is not better. And never pile it against trunks or stems; the famous advice is 'mulch volcano' is how you kill trees.
Bags or bulk delivery?
Mulch is bagged at 2 cubic feet (sometimes 3). One cubic yard is 27 cubic feet, so a yard is about 13½ two-cubic-foot bags. Under roughly a dozen bags, the pallet rack at the home center wins on convenience; beyond that, a bulk delivery by the cubic yard is dramatically cheaper per yard — and no plastic bags to deal with.
One quick planning tip
Measure each bed separately and add the totals rather than eyeballing the whole yard. Beds are rarely rectangles, but L-shapes split cleanly into two rectangles in about thirty seconds with a tape measure.