a tennis ball has a diameter of about 2.5 inches
so volume of a tennis ball is: 4/3 x pi x r^3 = 8.2 cubic inches ( estimating you could say 8 cubic inches
1 cubic foot = 1728 cubic inches
so you could fit 1728/8 = 216 tennis balls in a cubic foot.( estimated)
you would then need to calculate the volume of the room
lets say a small room is 9 foo by 9 foot by 8 feet high
9 x 9 x 8 = 648 square feet
now multiply the 216 by the square feet of the room
216 * 648 = 139,968tennis balls
now because they are round, there will be empty space around them
so you could estimate about 139,000 tennis balls would fit I a 9 x 9 room that is 8 feet high.
You didn't give the dimensions of the room, but this gives you the idea of how to estimate
