usb 6341 counter number of buffer

Hi
I will use usb 6341 to measure digital pulse.
What is maximum buffer when i use 25Mhz sample clock with buffered mode?
(It shown error when i set 25Mhz clock, N sample mode, 1M sample)

Is there any fast edge measerement with usb 6341?
(I want ns time scale with long time)

Thx