How many calculations per second can a computer make?

Depends on what you mean by calculations. An i7 3770k can perform up to 150~GFLOPS, or Giga-Floating Point Operations per Second. This number looks like this

150,000,000,000~ Floating Point Operations per Second. This is probably the number you're looking for!

As far for Instructions per Second.. This one is more advanced. The same i7 3770k, which is the processor with the highest Instructions per Cycle currently available in the modern CPU market, has an Instructions Per Cycle count of 3.5~

So, per second, this would be the following:

3.5 *3.5 Ghz= 12,250,000,000 Instructions per Second.