You will get one Art Stone T4 when maxing an Artifact 4, this Art Stone is then used to purchase an Artifact 5. Artifact 5 will be unlocked at the Artifact Shop as soon as you have obtained your first Art Stone T4.
The same apply for the Art Stone T5. You get one for each artifact 5 you will max, and you will unlock the Artifact 6 at the shop after obtaining your first Art Stone T5.
There are 18 Artifacts 5, you will get 18 Art Stone T5 from maxing the artifacts to purchase 6 Artifacts 6. To purchase more Artifacts 6, you will have to buy extra Art Stone T5 at the Arena Shop.
When installing the update 1.5, if you had at least one artifact 4 maxed or own one 5, you automatically get 20 Art Stone T4.