Explanation

Let prime(n) be the number of primes in the interval [1,n]. Then the prime number theorem states that

prime(n) ~ n/log(n)

when n tends to infinity. This means that

limn-> prime(n) /(n/log(n)) = 1.
de la Vallée Poussin

The prime number theorem was proved by Hadamard and de la Vallée Poussin in 1896.

Hadamard