The maximum sampling frequency for the ADC on the Expander Pi is dependant on several factors, the main two being the programming language and the model of Raspberry Pi.
On a Raspberry Pi 3 B+ using python you can expect to get a sample rate of approximately 13500 samples per second. Using C you can get a sample rate of 32000 samples per second. The C language is more than twice as fast because it uses a compiled program that has less CPU overhead than interpreted languages like Python.
Other factors can have an impact on the sample rate such as CPU load from other processes and the SPI bus speed so your maximum sample rate may be lower.
We have ADC speed test scripts for Python, C, C++ and Node JS which you can use to compare the performance of the ADC on your Raspberry Pi. All of the tests use our Expander Pi libraries which can be found on our Github repository at https://github.com/abelectronicsuk