What is "N" Number when plotting a graph using a PSD function and what value should i use?

I am analysing the response of a floating body in water over a a 3600 second period with a time step of 0.0125 seconds. I have a code which converts my data into frequency from time domain but do not understand what the term "N" represents and what value i should be using which is appropriate for my given data. I have 288,000 values of data (3600 multiplied by 1/0.0125). Please find the code attached.