There is actually J12, J13 in the early 90s, but canceled. J14 won't be used because it pronounces like "going to die" in Chinese, J15, J16 already in production, J17, J18, J19, I don't if there are so such programs or not.How so.
For the Z series, there are Z-9 to Z-19, I don't if there are Z-12, Z-13, Z-14, Z-16, Z-17 programs, But then Z-20
But for the Y series, There are Y-1 to Y-10, then jump to Y-20. That's why I am saying Y-20 breaks the convention.
If H-20 also exists, then H-20 also breaks the convention. Don't know why China so abdicated to 20.