math is hard..... ok, I've been over this on another thread. The average forest density in the United States is 5500 trees per acre. So 353 million trees equals about 64182 acres of equivalent US forest. This sounds impressive, but that equals 100 square miles. A square 10 miles by 10 miles in size.
For a little more perspective, Ethiopia is 426,400 square miles in land mass. This means they planted the equivalent of 0.02% of their land mass with trees vs the forest density of the typical US forest.

(for the record, those evil guys that cut down trees for profit, plant 52 million trees per year every year in the US)

Meanwhile, as per the 353 million-in-a-day estimate:
let's say for sake of argument that the average person can manage to plant one tree per minute and they are working an 8 hour shift with no breaks, pauses, distractions or complications. That's close to half-a-million people to cover all three shifts in a single day. And that's with generous assumptions.
The Ethiopian government says that they expect a single volunteer to be able to plant 100 trees in a day. Which means if the 350 million in a day is accurate, they had 3.5 million volunteers. Call me suspicious of such an ambitious estimate.

with a little more digging, to their credit, these 350 million trees are just the latest on a re-forestation project that now boasts 2.6 billion trees planted since it began. But math again, that equates to 739 square miles of equivalent US forest (less than 0.2% of the total land mass of Ethiopia or an equivalent square of 27 miles on a side of typical US forest. That's just over half the size of the state of Rhode Island or about the size of Washtenaw county, MI where I live.

Tt their ultimate goal is to plant 4 billion trees total before the project is finished. Admirable indeed. But, I did find figures on my home state of Michigan for comparison.
While I have yet to find a number on the total number of trees before the reforestation in Ethiopia, I can guess based on the decline of 25% cited and the goal of 4 billion trees total. ((4/25) * 15) = 2.4 (billion) for a total of 6.4 billion trees assuming their desire is to re-achieve the 40% number from a century ago.
Estimates for the state of Michigan are a total of 13.5 billion trees across the entire state. That's more than twice the assumed 'final goal' for the entire country of Ethiopia. Oh, should I mention the land area of Ethiopia is 4 1/2 times more than that of the state of Michigan?
Other things worthy of consideration which I noticed while digging. Many areas of Michigan were clear cut in the late 1800s and early 1900s. (i.e. about the same timeline as the de-forestation in Ethiopia) To be fair, some of the areas cut were re-forested by WPA government work programs in the 1930s. But not all. The reports I was pulling my figures from were actually surveys done by the state government for the purpose of evaluating potential future timber cutting, and they noted a large number of hardwoods (oaks, maples, elm, etc) in those numbers (most of the wpa planted trees were pine trees) and a growing quantity of older trees as a percentage of the total. aka "old growth" or mature forests.