What is after a heptalogy?

This is an Excellent question. The terms are as follows: Set of 2 - Duology, 3 - Trilogy, 4 - Tetralogy, 5 - Pentalogy, 6 - Hexology, 7 - Heptalogy, 8 - Octology, 9 - Ennealogy, 10 - Decology. After the 10 mark there is a slight change... 11 - Undecology, 12 - Dodecology... from 13 to 18 the same greek prefixes are used before the Word part "decology" ~ tri,tetra,penta,hexa,hepta and octodecology... on to 20 it is Nonodecology and Icosology. Hope this helps and answers your question.