A: Pages will be translated at the moment when a user clicks the language switcher and selects their preferred language.
So your word limit will not be used all at once.

By default, all pages, old and new ones, can be translated.
However, if you prefer, you can enable the Whitelist feature to translate only specific pages based on their URLs.
And opposite, you can exclude pages by URL, by partial matches in the URL, or exclude specific content by element IDs or classes.

If you reach your word limit, new pages will not be translated.

Please note: the word limit is not monthly, it is a global limit for your entire account.
If your Tier includes multiple domains, the limit applies collectively across all connected websites.