WordPress is a singular without plural. A second instance would be a fork that could not use the same name, because the name WordPress is a trademark of the WordPress Foundation.
Like Jesus, just more rules.
If WordPress had a plural it would be WordPresses, like in mess or mistress. But the prerequisite for that would be that WordPress becomes either a physical object or self-aware (and able to create self-aware copies). I will update my answer on the day that happens.