So it is just the directory in which the library resides that is still named "library 20".
So just back up that library, create a new library in "S:\Library", restore the backup you just made into the new library, and then delete (from within MC) the old library. Then use the new library in "S:\Library" from then on.
Of course, if you have any special customisations, scripts, or similar that point to "S:\Library", then will also need to update those. But those would all be outside MC.
BTW, if in the past you just renamed "S:\Library 20" to "S:\Library", then that would explain why MC recreated "library 20", as that is where its library was pointing to. It would have created that new library as an empty library until you restored a backup into it, or reversed the change you made.