How do I calculate sampling rate of external ADC?
Easy.


Based on your set up, figure out the spi speed.
Take a look at the adc datasheet, figure out it's frame of transmission . Aka how many pulses per frame / sampling.
The product of the numbers in 1 and 2 gives you the minimum time per frame. The inverse of it is the maximum transmission in your application. That speed is cap by the devices own max sampling rate (100ksps).
Your actual sampling rate will depend on your specific application but it should not go above the number obtained in 3.
You may need to factor in other timing requirements as well so read the datasheet carefully. But the gist is the same.