I started my garden with 2 or 3 raised beds because it was in a section of badly degraded soil and my neighbor had some 2x10 planks she was getting rid of. A trailer full of mushroom compost and a little topsoil and I was ready to go the first season living on the property.
However, the wood was old to begin with and in a few years I had to make a decision. New raised beds? Or in-ground? I chose beds that are corrugated tin or some kind of metal, with braces that go across the short way, for stability. I have been able to expand with more beds, some with scrap lumber and some with black locust logs. As they degrade, I have replaced with more of the metal beds. And my soil, after 10 years and countless barrows of compost and leaf mold, is now as valuable as my house. Well....I mean...you know what I mean.
Anyway, the latest garden expansion is on a slight slope and I decided to create berms and swales, not really deep or high because the slope isn't steep. I have started planting more perennials in the berms; maybe some annual veggies will be tucked in here and there. As the soil gets better, I will put in more berries and flowers and not sure what all.
So, it does depend on what you are planting, what your climate is, what you have to begin with. If you don't know, do a bit of dumpster diving/scavenging and get some free or cheap lumber and just get started with 1 or 2. Then you can decide from there how they work and if you want to make changes.