To calculate the remaining number of teams, use the sequence formula:
a(n) = a(1)Rⁿ-¹
Where a(n) and a(1) are the nth and first terms. R is the factor. Therefore, 24 teams is valid value.
After 5 rounds :
a(5) = 128*(½)⁴
a(5) = 128/16 = 8
8 teams are remained.