Prime number theorem - RSA
Apply your formula, perhaps simplified as $\ln(2^x)={x} \ln(2)$, so for 1536-bit numbers about one in $1536 \ln(2)$ are prime. In the case of $1{,}000{,}000=2^{20}$ (approx.) the number of primes is about one in $20 \ln(2)$.