I'm on a similarly sized team, and we have put more effort into automated testing lately. We got an experienced person in on the team that knows his shit, and is engaged in improving our testing. But it's definiely worth it. Manual testing tests the code now, automated testing checks the code later. That's very important, because when 5 people test things, they aren't going to test everything every time as well as all the new stuff. It's too boring.
So yes, you really REALLY should have automated testing. If you have 20 people, I'd guess you're developing on something that is too large for a single person to have in-depth knowldge of all parts.
Any team should have automated test. More specifically, you should have/write tests that test "business functionality", not that your function does exactly what it is supposed to do. Our test expert made a test for something that said "ThisCompentsDisplayValueShouldBeZeroWhenUndefined" (Here component is something the users see, and always exepct to have a value. There is other components that might not show a value).
And when I had to interact with the data processing because another "component" did not show zero in an edge case. I fixed the edge case, but I also broke the test for that other component. Now it was very clear to me that I also broke something that worked. A manual tester would maybe have noticed, but these were seperate components, and they might still see 0 on the thing that broke becase they had the value 0. Or simply did not know that was a requirement!
We just recently started enforcing unit tests to be green to merge features. It brings a lot more comfort, especially since you can put more trusting changing systems that deal with caluclations, when you know tests check that results are unchanged.