Thank you for your comments, W.
On the size of challenges, I ended up settling on lists of 14 people for formal challenges.
I have noticed that challenges with too few people just don't attract that much attention. (There was one post with three people in it posted in 2017, and another post with five people in it posted in 2019. The last time I checked, none of the people listed in those two posts had been connected yet.)
On the other hand, challenges with too many people in them take way too long to complete. The post asking people to help connect the people listed in a book called The 100 Greatest African Americans has 90 deceased people in it. That was posted in 2018, and the challenge still isn't even halfway finished. I posted a challenge in 2019 to connect astronauts, and ended up breaking that one into four separate challenges because it had just too many people.
The problem that I anticipate with asking a whole team to focus on one person is that it can lead to editing conflicts, unless people are coordinating through Zoom or some other instant messaging system: "Okay, I've done what I can. Who wants to try next?"
I agree that it makes more sense to focus on people who were born in the Netherlands, rather than people who happened to die there. If somebody from the USA, the United Kingdom, Canada, or some other country with good records died in the Netherlands (like during World War II), it only makes sense to look for their connections "back home", as it were. (Although it seems to me that people with Dutch parents, but who were born in someplace with, shall we say, limited records would also make good candidates for searching for connections within the Netherlands.)
And, yes, it is more challenging to work on older profiles, but as I understand it, Dutch records do go back before 1700, and, at least when I took the test, getting the pre-1700 badge was just a matter of demonstrating that you know the way things work on WikiTree. I have been so impressed with the work that people have put in on the Dutch challenges that I have posted in the past that I would anticipate that pretty much every member of the Dutch Roots Project who took the test would pass on the first attempt.