They mentioned Ninjas using Mudras (hand seals) you combine them in different orders, to cause different effects. if you dont combine them in the right order, nothing happens.
So there's 4-5 different seals, that you could make quite a lot of different effects with (potentially) So you dont really need lots of skill slots.
Also they said they will increase lv cap, when they add new jobs...so maybe we'll get lv60-75. in which case each job would get 5 more job skills, 6 class skills, and passive/traits.

Also...please stop using XI as model....
Most of the Jobs that were in XI, that are in XIV, work differently, so will Ninja.