Correct the maximum sampling rate is 250kS/s. This information can be found on the NI-622x Specifications.
Take a look at this discussion posting,
Aggregate sampling rate and Multichannel Scanning Rate, for more clarification on sampling rate. The sampling rate is going to depending on the number of channels you will be using.
Please let us know if you have any additional questions or concerns.
Have a good one!
Regards,
Sandra T.
Applications Engineer | National Instruments