Count them ...NINE

And she is still leaving the nest and not sitting on it regularly. I have noticed that she is putting more feathers into it though. Never thought about the expression "feathering your nest" to mean getting it ready to sit on for 28 days but it makes sense. My limited research tells me she will not leave the nest much at all after she is finished laying. Can you imagine being the size of a duck and carrying these nine eggs around?
Here's some of my research:

" Ma Mallard typically lays one egg per day until she has accumulated a full clutch. Early in the nesting season, a full clutch averages 12 eggs. Hens that lose the first nest will commonly nest a second (and even third or fourth) time later in the summer; these "renesting" hens generally lay fewer eggs, commonly 8-10. The first few eggs are laid in a shallow "scrape" in the ground. As laying progresses, Ma will add grass, twigs, and copious amounts of her own down to the nest, resulting in a well insulated and surprisingly well camouflaged nest bowl.
During the laying period, Ma visits the nest for only about one hour per day. While there she lays an egg and works on nest construction. Don't be concerned if she skips a day. She does not incubate the eggs during the laying stage. Because no embryo development has occurred, weather conditions in her absence generally have very little effect on the clutch. An exception is a hard freeze, which may kill the embryos if it occurs before the hen has insulated the nest. Once she has constructed a nest bowl, she will cover the eggs with nest material whenever she voluntarily leaves the nest. This behavior makes the nest less obvious to predators, and protects the eggs from temperature extremes.
Incubation begins after the clutch is complete, and typically lasts for about 25-29 days. During this period she will usually cover the nest and go off to feed for about an hour each morning and afternoon. As hatch date approaches, she will begin to feign injury if she is disturbed. This behavior evolved to lead predators away from the nest...once she is a safe distance from the nest, her "broken wing" will miraculously mend, and she will return to the nest as soon as the danger has passed. This behavior is fun to watch, but be aware that she is risking real injury in the process.
Because no incubation occurred during the laying stage, all viable eggs will hatch within 12-24 hours of one another. After a short period of brooding the ducklings at the nest, Ma will lead the gang off to her chosen wetland. This may be up to 3 miles away, so don't be surprised if you don't see "your" brood on the pond right behind your house...she will take them to where she has found the best source of aquatic invertebrates to feed them. Not uncommonly, one or more unhatched eggs will remain in the nest bowl. Some of these are infertile; others are "addled" (eggs in which a partially developed embryo died during incubation). Discard the leftover eggs, but do it carefully; gas buildup in an addled egg can cause it to explode when handled, and the results are not appetizing.
By the way…if you are wondering about Pa Mallard, he's off looking for an unattended female during most of this. He and Ma stayed together during selection of the nest site and the laying process, but as Ma began to leave him for long periods during incubation, he gradually lost interest and flew the coop. Not politically correct, but that's nature for you.