multi-db plugin application question


a general question here re Multi-DB, I see it is for multisite and spreading blog load.

I have a single site in development where I am expecting big membership numbers (BuddyPress Installed) and I'm concerned about the database size and managing a lot of members there, potentially 100's of thousands, so will Multi-DB be applicable, would it help or are there other ways to ensure that the membership can scale and the site continue to operate if there are a huge number of users, and simultaneous connections.?

I am setting up hosting on CloudWays, hooked into Amazon servers so hope that will help on scaling and reliability, but the database side needs some thought I suspect..

What else do I need to consider..?

thanks for some advice here..