I would suggest to stick with Postgres or MySQL unless you know why you need to choose something else.
If you’re trying to build a SaaS you will very likely run into a 1000 other problems before MySQL or Postgres will get in your way.
Personally I would recommend Postgres since IMHO it has more features to offer and good support in Ecto. If you’re already familiar with MySQL then it’s also fine to use that instead.
Instead of focusing on scaling I would focus on features offered, available documentation resources and compatibility. For ecto and Saas style apps Postgres is the first choice for me there.
Both Postgres and MySQL can scale quite well given enough hardware resources. As others have wrote, you will not hit limitations there soon. It’s very likely that app or database design decision you choose will become a problem before the actuall database engine itself.
So my personal recomendation is to go with Postgres (or MySQL if you feel more comfortable with that) and focus on start building your Saas.
I have used Postgres in various elixir side projects where it never got into my way and was easy to “set up and forget”.
But I also use it extensively at work where we use it not only for online transactional databases but also stuff like timeseries like databases, analytical use cases and datawarehouses.
While it’s possible the tune certain Postgres settings to reach peak performance it’s not something you need to worry or even know about until you hit those limits.
Feel free to reach out and ask more specific questions. I’m pretty sure you don’t need to focus on scaling but if you know what kind of database workload you’re planning for then a more specific answer might be possible.