wp-includes/pomo/translations.php:517Returns the plural forms count.
int public function get_plural_forms_count() { return 2; }Introduced in 2.8.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/pomo/translations.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.