I would say “certainly”, but as I don’t have a mil items in my app, it would be nice the get confirmation from others. But from a logical standpoint, yes. All non-dedicated users are on the same servers/db today anyway, so adding a million or so rows shouldn’t affect it much (a single PostgreSQL database can handle tens of terabytes of data - a million rows with a handful of standard text columns is about 200mb).
My app uses an external mysql database with around 10 mil items. I have built an api for bubble to connect with this, but I use this method mainly because I have large datasets that gets updated once a month and/or once a quarter. Importing 2 mil items in bubble is impractical, so therefore I have everything in a system of its own and connect to that with the api connector.
Edit: And as you point out, when you scale you might go to a dedicated server with your own database, and then these numbers wont matter at all.