Quite simply (but requiring more detail for other uses):
2^8 = 256
2^16 = 65536
2^32 = 4,294,967,296
2^x = 2 to the power of X-bit.
So 8-bits are simply 2 to the power of 8, which gives you 256 decimal numbers. You’re basically doing number conversions, similar to how you would convert pounds to kilos.
These numbers incidentally, are what some people call “special numbers,” because they appear everywhere in computers. Port numbers for instance, are 16 bit. (You can have port numbers from 1-65535 — port 0 is reserved)