2 sides are counted as 1 page or is it per side is 1 page while measuring a printer's output? So if someone says that X printer gives 200 pages output per cartridge does it mean 200 pages of single sided paper or both sides are counted?
It's 200 sides. The computation is done assuming that 5% of an A4 sheet is completely filled with ink. Don't bother about this detail if it bothers you. It just means that for normal printing you will be able to print 200 single sided sheets, or 100 two sided sheets.