Well you were talking of an alphabet, so I presumed it was our English alphabet so there would be automatically 26 to the 'series'.
Series CAN be open ended. Editions should not be for ACEOs.
You are right re the cats.
'Garden Cats, Series I' (might consist of 15 cards)
'Garden Cats, Series II' (might consist of 3 cards - if your muse dies, it dies
🙂 - could be 45 if your muse thrives! HA!)
etc.
... is fine, without stating the number in the series.
It can sometimes take an artist years to do a series, that is why it can't be estimated like with prints.
~Jillian
artist, Jillian Crider
... google me!