Is there a definite ruling on what constitutes an edition?
There isn’t a legal definition of what makes an edition.
Instead, an edition may be limited by contract (which can be verbal) and by making representations. So, for example, if the edition is said to be 8 plus an artist’s proof, it would be fraudulent to then make a 9th.
When nothing is said about the number in the edition or nothing is said about the limits of that edition, for example whether the edition only refers to works in that material or of that size, there isn’t a definitive answer.
Best practice would be to record details of the edition in writing and for each work in the edition to be clearly numbered, and accompanied by a certificate stating which number it is in the edition.