If you want to learn and complete the entire Imperial Plate Set, you will want to have 114 Thorium Bars, 12 rugged leather, and 1 star ruby.
You need to be level 50 and have 265 Blacksmithing. You do not have to have chosen a blacksmith specialty yet. You will want to collect 135 Thorium Bars to turn into Derotain Mudsipper for the quest Imperial Plate Armor
given by Krathok Moltenfist in Orgrimmar for horde and Myolor Sunderfury in Ironforge for alliance.
The patterns you will learn for the Entire Imperial Plate Set are:
Imperial Plate Belt - 10 Thorium Bars to learn pattern (BS:265) Level 47
Imperial Plate Shoulders - 10 Thorium Bars to learn pattern (BS:265) Level 47
Imperial Plate Bracers - 10 Thorium Bars to learn pattern (BS:270) Level 49
Imperial Plate Boots - 20 Thorium Bars to learn pattern (BS:295) Level 54
Imperial Plate Helm - 25 Thorium Bars to learn pattern (BS:295) Level 54
Imperial Plate Chest - 30 Thorium Bars to learn pattern (BS:300) Level 55
Imperial Plate Leggings - 30 Thorium Bars to learn pattern (BS:300) Level 56
Good luck getting all that thorium and skilling up your blacksmithing.