I guess this is more for people who are already in the middle of reading the series.
Most of my series usually have 9-10 novellas. Usually being round out to 3 novels. Though someone has expressed interest in me continuing past that normal cut off date.
I am kind of mixed on that idea, since the reason why I tend to write series like that is so I can move on to new ideas without being linked to the same cast of characters.
It also drastically lets me change the designs of characters since a new series doesn't really have to follow the same design principles for character designs.
If I do continue the series past 10 I'd probably want to set up a new cast of characters anyway, thus making the numbering kind of pointless.
Who knows, maybe I'll continue with the same cast and just write a similar series on the side.
Regardless, it's probably a bit too soon to worry about that since I'm still in the middle of book 4.