@icq70610 - my very first calibre library I tried to do something like you mention as I too thought it would be useful as a visual reminder that there were "more books coming" or I had them all. In my case I put the number of books in the series name in parenthesis, such as "Lord of the Rings (3)". However as pointed out it only works where that number is known and not for open ended series. Or equally, for series where you "think" they are complete, but then the author decides to throw another book into it (sometimes many years later). Heck even an author's death doesn't always guarantee the end of a series.
So once I realised that since I couldn't "rely" on that series count number, in combination with the ongoing maintenance to update it on every book in the series I just dumped it. Instead I use a combination of creating empty books as wishlist items for a visual reminder of upcoming or missing books, as well as just a quick flick via a keyboard shortcut of my Search the Internet plugin to bring up the author's page at Fantastic Fiction if I have any uncertainty about whether a series is complete or not.
If anything I think rather than a series count, some sort of flag at series level to mark a series as complete or not would be more useful. That way you don't have to update it on every book every time it gets added to. Of course that would then beg the question of how to visually represent it, like an asterisk after the series name or whatever.