How does a computer use just a few registers?

Question

I know a little about assembly, and that there are 4 or 8 or so general purpose registers. How do all the programs on a computer work with just that amount of registers, especia

Answer 1

The other variables and thread stacks are usually stored in protected memory space, where they can be called into registers when needed.

You may want to check out the book The Elements of Computing Systems for a good understanding of how your computer's CPU works. The book is set up as a series of projects where you work up from a NAND gate to a CPU, assembler, simple compiler, and on to a small operating system. It's invaluable in understanding how all your computer's parts fit together.