Jewelry is not counted towards any passives for weights of armor.
So despite my necklace of Ebon Armory belonging to a "heavy armor set", it does not count as a piece of heavy armor. It will still count towards 5 pieces of the Ebon Armory set.
Almost every build but a new leveling build will have at least 5 pieces of either Light, Medium or Heavy armor as there are many 5 piece passives in the game that only trigger when you have 5 worn.
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PvE Stam: full 7 medium; some experiment with 6 medium / 1 heavy chest for punishing non-trial content
PvE Mag: 5 light, 1 heavy, 1 medium. Typically the Heavy/Medium pieces are the monster set as useful dropped sets only come in Light. Though Prior to get the undaunted passives, 7 Light is technically better (I think)
PvE Healer: same as PvE Mag. Though if using Torug Pact can make chest heavy/legs medium for extra resists. If using Ebon or Hircine with a special staff 5 light with 2 Heavy/Medium chest and legs + Jewelry is another option
PvE Tank: 7 Heavy, 6/1 Hvy/Light, 6/1 Hvy/Medium, and 5/1/1 are all valid. There are also 5/2 Medium/Heavy builds that work well for either damage tanking or getting access to major evasion as a non-NB. 5/2 Light/Heavy is not common but can experiment with for running as a tank/healer hybrid with 3 DPS
PvP is rather open. 5 of one type is key. Crafted set use permits having higher armor levels through slotting the higher resist pieces (Heavy>Medium>Light) as Chest and then Legs.