There are an infinite number of prime numbers, but between 1 and 100 there are 25:
2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,97
That's 25%. Between 1 and 50 there are 15, but between 50 and 100 only 10; so the proportion of primes decreases as we go to larger numbers.