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.

Generally speaking for most players:

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.