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

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

To clarify, I don't think the Watchtower score itself is useless, but the number is rather subjective based on factors that aren't public, so I guess the user can decide how valuable it is to them specifically. The comparison against a credit score is the best analogy I could relate to it to. Credit providers aren't looking at the score you see, they are looking at the markers on your history. So in this case, the markers like no re-used passwords, strong passwords, 2FA etc are all the factors you should focus in on directly and less around the number.

Here's one example of why that number could be somewhat subjective. Watchtower uses 2fa.directory to match sites with 2FA support, however they have a hard rule of only sites that are globally ranked as 200,000 or better i.e. lower. That means by extension, Watchtower won't match on some sites which have 2FA available because 2FA directory won't ever list them. Now without knowing the various criteria, if the 2FA scoring relates to 2FA.directory data, then that's been somewhat constrained by that project's rules.

I personally think the password strength indicator is possibly better than a score, as it immediately identifies less complex passwords vs the stronger and shows where you might want to improve, simply by regenerating a new password. It does break these down into categories, which again could be argued subjective i.e. what makes a "Fantastic" password vs "Excellent", length, characters used, symbols most likely, but it does show you where weaker categories like Weak, Fair, Good or even Very Good are hiding without individually going through each vault item manually.

My point, look at the markers, i.e. password strength coverage across all sites, no weak/reused passwords, 2FA enabled on every site where possible (including one's what Watchtower won't ever be able to notify you directly about), don't get fixated on score which despite it's good intentions and purpose, is subjective at the end of the day.

I do like Watchtower, its intentions are good but it does have a potential risk of false sense of security in some places if not carefully understood.