What is the maximum amount of ripple input voltage a linear regulator can handle?
The LM7805 datasheet (https://www.sparkfun.com/datasheets/Components/LM7805.pdf) provides a very clear spec for ripple rejection for VI = 8 V to 18 V, f = 120 Hz, which is minimally 62dB.

If there is ripple near those parameters on the input, so long as none of the other parameters are exceeded, output ripple will be at least 62dB smaller.

If you need something more exact, or smaller, you need to find a better datasheet, or more likely, a better part.

The ST version of the LM7805 (https://www.st.com/resource/en/datasheet/l78.pdf) has a similar spec, "Supply Voltage Rejection", of 61dB, and they also supply a graph that suggests you should be able to rely on that behavior up to around 10kHz.

As for "maximum" input ripple, I'd just assume that you can expect the above behavior so long as you respect the maximum (and minimum) input voltage specified.