What is the meaning of the number in the upper-right corner of the Watchtower screen?

Ultimately, it's just a visual indication of your password health and 2FA coverage and the number itself is meaningless, much like your credit score represented as a number.

That's certainly one perspective. I'd say it is a good visual indicator to use for the relative health of the data in your account. If the number is going up, you're making improvements. If it is going down, it may be time to evaluate your practices.

That is really what it is there for: a benchmark that you can work toward improving.

Ben