Science is like a rainbow. How many different colours are there? A few main ones, but when you zoom in, you realise that each of those main colours (or areas of science) can be split down into hundreds or thousands more, and more at the edges of where you’re looking as well. Think of biology, there are thousands of sub-areas within biology, but also several that are on the edge of biology with, say, (bio)chemistry, or (medical/bio)engineering, or cybernetics, or psychology.
There are probably as many areas of science as there are scientists working in them!