NI-DAQ™mx C Reference

I/O Type >> Analog Input >> Timing >> Maximum Multiple Channel Rate

Data Type: float64
Description: Indicates the maximum sampling rate for an analog input task from this device. To find the maximum rate for the task, take the minimum of Maximum Single Channel Rate or the indicated sampling rate of this device divided by the number of channels to acquire data from (including cold-junction compensation and autozero channels).
Restrictions: Not Settable

You can get this property using:
  DAQmxGetDevAIMaxMultiChanRate