Will a single subscription apply to all my devices?

Yes!

All Wokabulary Unlimited subscriptions are universal purchases. A single subscription will unlock Wokabulary Unlimited on all devices linked to your Apple ID.

So when you purchase Wokabulary Unlimited (Flex, Pro, or Family) on your iPhone, this will also activate Wokabulary Unlimited on your Mac, iPad, and Apple Watch. (And the other way around as well, of course.)