I didn't know a database could hold billions of rows and still work fine. Good to know. That answers my question. Of course I was giving an extreme example when I said millions of rows. But if it can handle that, then great. I was a little hesitant before because I never had that many records stored in a table before.
I had a little side project a few years ago downloading achievement data from the world of warcraft api to run some statistics on it. If I remember correctly my table linking characters to achievements and their completion dates got to about 8 million rows before mysql started to struggle with it. Adding partitioning
let me get up to 12 million rows with mysql still handling it like a champ. This was on a fairly small VPS as well (20GB disk, 2GB RAM iirc). Ultimately I stopped around the 12 million mark not because mysql was struggling but because my VPS just didn't have enough disk space for the data and I didn't want to invest more money into it.
Moral of the story is you probably shouldn't worry about how many rows you may be adding to a table. Focus on a proper database design and worry about the performance if you actually have a problem.