I would vote for "two" but in answer to the actual question, "How many pit stops at Singapore?" I feel it will actually be too many and if we were to see all of them we wouldn't see any on-track action at all. So , two.
So with 21 drivers at 4 stops each and 3 drivers with 5 stops each there could be 99. Good thinking, it fits the wording of the question perfectly.