Q. How many instructions in a Computer?

A computer can have a vast number of instructions and addressing modes. Older computers with the growth of Integrated circuit technology have a very complex and large set of instructions. These are known as 'complex instruction set computers' (CISC).  Illustrations of CISC architectures are Digital Equipment Corporation VAX computer and IBM 370 computer. 

Though later it was found in studies of program style that many complex instructions found CISC aren't used by program. This lead to the concept of making a simple but faster computer which could execute simple instructions much faster. These computers have registers addressing,simple instructions and move registers. These are known as Reduced Instruction Set Computers (RISC).