3 plating for epic (it goes 1, 2, 3, 4). Last I checked (PC/NA), purple platings were going for around 20K, so probably not worth it from a gold/voucher perspective.
So with the latest update, how many platings does it now take to upgrade jewelry? I got a master writ to craft an epic ring, but I only have 5 zircon platings so I question whether it's worth bothering with. The master writ is worth 34 vouchers.