What's the number of FFTs in Welch-based PSD?

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?