Learn English – A piece of paper and not ‘a paper’

As @FumbleFingers points out either is acceptable. I think the distinction is very slight but present and depends on the point the author was making.

If he had used "developments" that would have called attention to the fact that there are several different developments that make up the outer portion of the area. I would have used developments, therefore, if my point was about the fact that these multiple independent developments ultimately cost more than if a single developer had been contracted to develop the whole area (for example).

In this case, though, I think the author was less interested in talking about them individually and just wanted to focus on them collectively. I think the author's point was in the low-rise nature of all the development in that area regardless of who had built which parts. Thus development was used.

Your base sentence is itself ambiguous.

1. It may mean that each of the tasks with which you are concerned outputs a single new task.

2. It may mean that each of the tasks with which you are concerned outputs multiple new tasks.

3. It may mean that the tasks with which you are concerned participate in an aggregation or collaboration which outputs multiple new tasks.

4. It may mean that the tasks with which you are concerned participate in multiple aggregations which each output one or more tasks.

Context will ordinarily sort out which of these is intended, so in practice most people will find all three sentences you put forward acceptable (provided you replace the singular verb outputs in the third one with output).

But if you are concerned to avoid the consequences of the Adamantine Law^† you will compose a different sentence for each. Given your preference for expressions with whose rather than which, I suggest:

1. Generally speaking, this concerns tasks each of whose output is another task.

2. Generally speaking, this concerns tasks each of whose outputs are more tasks.

3. Generally speaking, this concerns a set of tasks whose aggregate output is more tasks.

4. Generally speaking, this concerns tasks participating in aggregations each of whose output is one or more tasks.

Myself, I prefer to replace the expression each of whose with whose each. However, a discussion of whose each on our Elder Sister Site suggests that this expression is unfamiliar to many contemporary readers; so it should probably be avoided under the Insularity Principle^‡.

^† The Adamantine Law: “Any expression that can be misunderstood will be.”

^‡ The Insularity Principle: “Any expression, however well established, will be rejected as ‘ungrammatical’ by those unfamiliar with it.”