I like it when decor is
functional. I prefer a natural layout of things which one would find in the natural habitat of the fish. I've seen some brilliant aquascapes on YouTube but they are +70% plants/hardscape - which is great for a work of art but it seems to diminish the fish! I got a fishtank to look at fish, not rocks

My tank is probably 40%-50% decor (to look at, not by volume

) but that consists of thin spidery wood, tall plants along the back and small plants at the front, so there's lots of shelter and decoration but the fish are still the main attraction - I can see them all pretty much all the time (except the pandas).
Based on my limited experience as a newbie, I would suggest that you browse the net for a "look" that you like, and select your decor based on that "theme". When I started out I bought bits of everything and when I tried to put it all together it looked a mess. I also wasted money on fake plants that I ended up not using. Less is more; stick to a few colours and substances and build the aquascape up. Stop when it looks right. I don't think there's a hard rule, but obviously the greater volume of decor you use the less water you will have. That's why live plants are the best!
