Normally for a 12 bit DAQ device the maximum sampling rate of a SCXI chassis is 333 kS/s. There are several factors that affect this such as number of channels, DAQ device, number of chassis, etc. Here is a link that contains some additional information.
http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/410a70c25a4d12b486256a1e0070bdae?OpenDocument
I hope this helps.
Joshua