To add to that explanation, you can prove that the number of people who can be simulated on a halting n-state Turing machine has no computable upper bound by considering a Turing machine that alternates between some computation and simulating humans every fixed number of steps. If we could compute an upper bound on the number of humans simulates we could compute whether the TM would halt by waiting for that many people to be simulated, similarly to how we could use BB(n) to determine whether any n-state TM will halt if we knew its value.