Each weaponskill gives 5 Ninki. At level 78 your finishers and Shadow Fang also generate 10 Ninki (On top of the 5 they generate baseline from the level 62 trait).

This means, that at base, you're getting 15 Ninki per combo, then after 78 you're getting 25 per combo. Meaning every other combo should yield a Ninki skill.