how to calculate sampling rate for DAC from its data sheet

A DAC does not have "a" (one single) sampling rate. It depends on how you use that DAC.

Let's look at the datasheet of a common DAC, I choose the MCP4725.

I just start reading on the first page.

This DAC is a 12-bits converter

It has a typical settling time of 6 us => Aha, so it needs at least 6 us to "settle down". That means that the time between samples can never be less than 6 us. If we assume we can get a sample every 6 us that means the sampling rate would be 1 second / 6 us = 166 kHz. That's the absolute fastest this DAC can do. That is also without any margin so the real speed will be less. I personally would not use this DAC above 100 kHz so 100 kSps.

Then we need to get the data into the DAC and there might be a limitation there as well. The fastest datarate this DAC can do is 3.4 Mbps (Mega bits per second).

The data is send to the DAC in chunks of 2 bytes so that is 16 bits. That means at a datrate of 3.4 Mbps / 16 bits = 212.5 kSps. That's 212.5 kilo samples per second, more than the 100 kSps I recommended above so the datarate of 3.4 MBps is enough to support the 100 ksps.

However at the standard datarate of 100 kbps things will be different, then 100 kbps / 16 bits = 6250 sps (samples per second). The datarate is now so low that the sampling rate must be lower, it is limited by how fast the data is coming in.