The number of prime numbers not exceeding a given number '''n''' is a mathematical function, P''''''rimePi[n] in WolframMathematica notation; it equals approximately '''n/(log n)'''. Conversely, '''k'''-th prime is approximately '''k*(log k)''' in magnitude (very roughly, but sufficiently so for asymptotic complexity calculations). 

For more precise formulations look up PrimeNumberTheorem, e.g. http://en.wikipedia.org/wiki/Prime_number_theorem.