When i am in forge and select an object like 2x1 block and the number next to it(how many of them pieces left) is everything else meant to reduce as well
Yes. The 100 limit next to building blocks, the 50 limit for walls etc. are the limits that encompass the whole category.
Why is that? If a map requires a large amount of a single category, say walls, you hit the limit very quickly.
This is just a guess but because then the limit on individual objects would be much smaller in comparison. Like you probably wouldn't be able to place like 20 Colosseum Walls. Some objects are use more than others depending on the map and object. I prefer it this way, don't have to use things as sparingly.