When do they add new statues?

Each time they release a new OS.

Each time when new Android OS is available, they add a new statues in Googleplex plaza, and move the previously one to this lawn. When Android Pie is released, the Pie statue is at googleplex plaza, and Android Oreo statue is moved here.

Yes, each time a new Android Os release

Whenever new Android version got released. New statue may be in Google Headquarter.

Oreo is there, but Pie is not as of Oct 2018. New releases are kept elsewhere.

Each time on introducing new OS..

Not at the android park