With the question I made about primes, I noticed people enjoy the subject, so here's another thought: let k be a positive integer; how many primes are there from 1 to k?
There's probably no exact expression for (let's call it) N(k) — and, unlike the other question, asking for $\lim_{k \to \infty} N(k)$ is silly because it's obviously infinity —, so, in a lenient variant of the question, does N(k) asymptotically reach a certain function of k as it becomes larger?