I've read from the book "Proakis, J. G., & Manolakis, D. G. (2004). Digital signal processing. PHI Publication: New Delhi, India." Chapter 12 that the number of FFTs required for estimating the PSD from Welch method with 50% overlap is $2N/M$ where $N$ is the amount of data that you have from the signal, $M$ the size of the segments you choose for the method and I guess the $2$ is for the 50% overlap but from the articles I've read they usually do one FFT less than the one described in the equation, why is that? or what am I missing from this interpretation? My theory is that maybe the FFT missing it's supposed to be done with the last 50% of the data and another 50% of data completed with zero padding. Am I correct?
The use of the fast Fourier transform in power spectrum analysis is described. Principal advantages of this method are a reduction in the number of computations and in required core storage, and convenient application in nonstationarity tests. The method involves sectioning the record and averaging modified periodograms of the sections.
It clearly states the minus-one version: