This is more complex than it sounds.

Project watchers get notifications while they are watching, and they stop receiving them when they stop watching. If we add them to CC, they will be tied to the tasks where they are CCed until they unsubscribe manually. If we don't subscribe them, only mention them, then we might be creating the wrong expectations, because watchers are not subscribers.

Then still the list will need to be kept up to date during the life of the task. How many watchers had the project when the task was created? How many will receive a notification to my comment posted four years later? Which number to show?

Then there is also the question of a task associated with several projects, having to merge the list of all names.

Projects can be CCed, which means that members of that project will receive the notifications as well.

For what is worth, there are also Herald rules (T360), which are private, which can send notifications about events to users not being subscribed.

Too complex and too much work to presenting a number that, at the end, doesn't bring much. If you want to be sure that a person or a team is following a specific task, CCing them is better.

I don't think upstream is going to be interested in this feature. Proposing to decline this task.

I think this is about simply linking to the list of project members and giving the current number of them. Rather than adding each one to CC.

I know that some people are watching the project, though. Could we show them, for the sake of task reporters understanding what's going on?

Can you elaborate on "understanding what's going on" and why you assume that something actually is going on? :D
I understand that people feel better if they see a random other person subscribed to an issue and it is also more likely that a comment is read by a random other person when there are 100 subscribers instead of zero.
Still I think it is a wrong assumption to expect that subscribers actually read all their notifications. I'm CCed on everything but I don't read everything (due to my local mail filters).

So if it was only about "I feel better if some random person is on the CC list and hence receives a notification" I'd rather prefer to not expose the CC list at all for the reason given above.
But there's also the valid case "I want to make sure that a specific person receives a notification by making sure the person is on the CC list" which is my only reason why a visible CC list is actually useful. But that's unrelated to whether that person watches the project or not anyway.

I think this is about simply linking to the list of project members and giving the current number of them. Rather than adding each one to CC.

This quote from one of the comments above is correct. I am not suggesting to add all project watchers to CC manually.

All I am suggesting is to show a "and NNN other people watching the projects" phrase in the web interface, which links to an accumulated list of watchers of any projects the task belongs to. Nothing would change in terms of how mail is delivered, this is only a request to have the interface display the current situation without altering it.

Project members and watchers are different concepts. Relating these users with a CC field can be misleading.

If the user wants to have an idea about who will be notified about a change in a specific task, they can click the project button(s) and see by themselves.

I don't think upstream will accept this idea, but I would be wrong.

Project members and watchers are different concepts.

I didn't even mention members. I only mentioned watchers, who are listed at the bottom of emails. :-) Here is the footer of the email I got for your comment:

To: Qgil
Cc: Aklapper, Gryllida, Krenair, Qgil, jeremyb

jeremyb is on this list only because they are watching the project. What I'm suggesting is in the middle somewhere:

Relating these users with a CC field can be misleading.

Depends on the implementation, really. What I do find undoubtfully misleading, though, is listing only me and Aklapper on the CC field -- this way, I get a feeling that Aklapper might be

While I appreciate Aklapper sitting in such prominent place and volunteering to take requests from people who file new tasks, and to triage them, I feel that "a click and an attentive read away" (with most users not even knowing that project watchers exist, and assuming they don't) is too much effort for learning who really is watching the task.

I get a feeling that Aklapper might be

Heh. Sorry, but I am just subscribed to your task. That's all, and that's why the field is called "Subscribers". :D