Thank you for your kind words and welcome to the group!
On suggestion of my friend I looked up a.t.tha in the dictionary, and
it turned out that 8 (as well as 4) has certain subjective
significance.
T.W. Rhys Davids writes in PED:
(I) As mark of respectability and honour, based on the idea of the
double square ...
(2) As a number of symmetry or of an intrinsic, harmonious,
symmetrical set, a.t.tha denotes, like dasa [10], a comprehensive
unity. See especially the compounds for this application -a.msa and
-angika. Closely related to numbers 2 and 4 a.t.tha is in the
geometrical progression of 2. 4. 8. 16. 32. where each subsequent
number shows a higher symmetry or involves a greater importance.
So I think the number of parts in A.t.tha.ngika Magga has both
practical and 'harmonical' reasons, and 10 is also a number which
denotes comprehensive unity.
Remember popular 7+-2 approach in modern psychology and New Age?
According to it, if a quantity of items is between 5 and 9, they are
grasped more easily by human mind.