Plot data to Export - Number of data points

Hi all,
How can i determine the number of data 'points' (for example in txt) of an exported plot?
For example, the velocity profile of a fluid flowing inside a cylinder(with D diameter) , may have a curve of n points(in excel) for the D or n+m for the same distance/Diameter.. in other words, the step moving from 0 to D differs and consequently the number of data points(finally i get with txt at excel).