I'll take a page from quantum mechanics and only generate the ones the player actually looks at.
In fact, if you subscribe to the 'the universe is a simulation' theory, that is probably how quantum mechanics was designed, to cut down on file size 
Just an aside, but that’s a common misunderstanding of quantum mechanics. No quantum interaction needs a person around to observe it before it happens. Quantum experiments often talk about what is “observable”, or even mention “observers” and “observations”, but they don’t mean a human is watching things. All they mean is that the interaction has a measurable effect, and that measurement involves _any_ interaction between particles. That interaction could just as easily be with a stray radio wave passing through your experiment, or a cosmic ray, or a random particle from the environment, etc, etc. Quantum experiments are usually very noisy. They are repeated many times per second, and the results that don’t match the expected values are discarded or counted separately. This is why the apparatus is often so complicated; putting the experiment in a vacuum, chilling it to extreme temperatures, trapping individual atoms in beams of light, all of that is designed to decrease the noise and increase the number of successes that you get.
But no one will mind if the game doesn’t bother to calculate things before they are really needed.
