Learn English – >too many efforts – is this correct gramatically

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.

They are both correct, given proper context.

If you want to ask someone how many whole onions, they want, you use the former:

A: I need you to get onions from the store.
B: How many onions do you want?
A: Three or four, please.

Note that the answer can be fractional:

A: I need onions for this recipe, can you get some from the store?
B: How many onions does the recipe call for?
A: One and a half large onions, so if you could buy two, that would be great.

This is the more common usage.

However, if you want a volume of onion, either chopped or prepared in some way, you would use "much".

A: This recipe calls for a lot of chopped onion!
B: How much onion does it need?
A: Three cups!