In the immediate, yes there's a very finite number of jobs. If there was an unlimited supply of jobs, not a single person in the world would be unemployed.

Jobs are tied to the economy, until the economy grows there can't be more jobs. The system might handle a flux of 10% new workers (either from a bizarre immigration mistake or a ton of people working over time or double full-time jobs) however, resources are finite. All 300 million people in the USA can't overnight get a second job.

In certain sectors there's probably great ease in getting a second job. However, there's not 300 million minimum wage jobs going.

I agree with that point, and perhaps I was a little callous in my original (grandparent) comment. The demand for workers is often very inelastic because it requires someone to figure out new ways to add value that requires workers.