Answer : 5525 Balls

Start from the top of the pyramid.
For a pyramid of 1 level - Number of Tennis balls required is 1
For a pyramid of 2 level - Number of Tennis balls required is 1 + 4
For a pyramid of 3 level - Number of Tennis balls required is 1 + 4 + 9
For a pyramid of 4 level - Number of Tennis balls required is 1 + 4 + 9 + 16

How do you go about doing this is each level of N balls requires N+1 balls as support in the case of 2 dimensional pyramid, since this has 3 dimensions, each N² ball requires support from (N+1)² ,
therefore there would be N² Balls in N^th level where N is 1,2,3,4... 25. So the answer here is

1² + 2² + 3² + 4² + 5² + .... + 25²

Using this formula to calculate the sum of squares of first N integers , we can calculate the number of balls requires as

(25 x (26) x (51) ) / 6 = 5525