There are several different versions of "in series". One is two complete phasers, that each combine phase-shifted and dry signal, such that there are two intact circuits, each producing 3 notches. That, in turn has two different subforms, like I illustrated in my video: one using a common LFO, and one using two independent unsynced LFOs. The second form of "in series" would be one in which the first set of 6 phase-shift stages are placed in series with the second set (12 stages in all), and THEN combined with the dry signal, for 6 notches.
Personally, I've come to view 6 stages as the sweet spot for guitar phasers. Much more than that (okay, so MAYBE 8 is fine) and the notches become situated outside the frequency range of normal guitar signal. I mean, it will sound "different", because the notches you can hear are moved over in the spectrum, but it won't sound like more notches. It will be good for keyboards, and other instruments that have broad bandwidth, but not especially inspiring for guitar; at least no moreso than 6 stages. But that's my view and not gospel of any sort.