Arduino MKR analog maximum frequency input
Under the right circumstances the ADC on the SAMD21G can do a maximum of 350ksamples/s.
There's more to it than just the number from the datasheet though: when using the standard Arduino libraries you will get nowhere near that number, mainly because of code overhead and because they set the ADC to a whopping 63 clock cycles per sample.
I put some example code for faster settings for the SAMD21G's ADC here.