Its 2.
divide it into groups of 3, 3 and 2.
Weighing 1.
weigh the 3 and 3 groups against each other.
(i) if they are equal , the heavier ball is in the group of 2.
(ii) if one side is heavier, the heavier ball is in the heavier group of 3

Weighing 2.
if its (1) above,.take the remaining two to know which is the heaviest.
if its (ii) above,take two balls and weigh agaisnt each other, if neither is heaviesr, the third ball is the heavier one; if one of them is heavier, its the heavier ball and problem solved.