Well, there’s a “serial” in the sense of individual stories and a serial in the sense of individual acts. Acts other than the last one should – maybe even have to! – end on a point that makes a reader want to know what happens next. In a TV show, generally the act divisions run from commercial break to commercial break. (TV shows often start with a “teaser” before the first act and commercial break and very often end with a “tag” after the final act; in series that have long arcs, the tag usually delivers some kind of weird twist that advances the arc story.)
So: it sounds like you’re talking about complete stories that are related, right? I suspect in that case the feedback you’ve gotten is probably all correct. Series do tend to draw in readers, and everyone I’ve talked to has suggested that having more work available for purchase has a cumulative effect: if you have ten books available for purchase, you’re probably going to sell more copies of all of them then you would if you had only two, because people who like one book in a series will very often buy all of them as long as that’s not too expensive to do. But the catch there is of course that the more books are available the more expensive buying all of them is, and unless you kill your own series on a high note, I imagine sales will start tapering off.
Of course, if you’re talking about releasing stuff for free, that’s a bit of a different animal. I imagine reader fatigue and disappointment can still set in the longer any series goes, but cost stops being a concern.