A standard sized helium balloon has - if you look from the side - a vertical diameter of 30cm (measured from top to bottom).
But if a helium balloon is hanging against the ceiling and you look from below, the balloon only has a horizontal diameter of 25 cm.
This means that there are a maximum of 4 rows of 4 balloons = 16 balloons in 1 square meter. Well, then there is no longer a piece of ceiling to be seen.
As a rule, if you calculate with 9 balloons per square meter (m2) - so 3 rows of 3 balloons - the ceiling is nice and well filled.