Now question, why does Saiyuki only have 2 ID's? Basically because the series is not listed for more chapters after that. What I mean is, MANGA Plus saves 1 ID per Shonen Jump issue the series will appear on, so if they have 4 ID's for Yui Kamio, it means the series has appeared in this week's issue, #36/37, and hence will appear on Issue #38, #39 and #40. After that, it's completely unknown whether the series continues or not, that's not something we can calculate because when they relocate the series, they tend to use random ID's and leave huge spaces between every issue.
So, if any series has less than 4 ID's, does this mean the series is ending? Of course not. Series could be having a break, hiatus, or any other thing that could mess up its schedule and not appear in some issue. But of course, we can just guess the series its ending due to the overall knowledge we have about it.
That should be everything. I hope this helped to understand how the whole system works.