For 40k visitors I assume you’d be building your application on the Maker plan. Note that users will be able to sign in to your app with their personal email only.
As for the number of users and concurrent users in particular, to be confirmed, but I’ve heard/read @david, the CEO of Glide, mention multiple times that Glide applications can handle that type of volume and more.
To be on the safe side, you might want to use Glide tables to build your app and perhaps avoid Airtable or Google Sheets to stay clear from sync delays, especially to handle data the visitors might need in real-time.
If people don’t need to log in, then they won’t even be considerer users (a person who signs in with an email address), only visitors. In that case your Glide application will be similar to a public website. My guess is that Glide’s servers could handle that kind of volume. Guessing is probably not good enough, you might want to wait that others chime in or do a advanced search in the forum, there have already been discussions about volume.