> If several (in fact more than 10) of my bookmarks share the same
> web-based login, can I set it up so that there is only one login
> instead of as many logins as there are bookmarks?
Only if that page has a "Stay logged in" option which saves the login information in a cookie. In that case you have to login once in a browser tab of WebSite-Watcher, this will write the cookie which will then be used for the bookmark checks.
If it's not possible to save login information permanently, then you have to record a check macro for each bookmark. In that case it's not possible to share the login information.