Ball change in ATP tournaments

[gahaha]

Is it every 5 or 7? Google said 7 but I swear a lot of the matches it seems like 5, or am I mistaken?

Oh and they use 6 balls at a time?

 

[gahaha]

Help anyone?

 

[Bobby Jr]

They change after the first 7 and then after every 9 from there on in.

The first change factors in the warm-up.