db.inventory.updateMany(
  { "tags": { $exists: true } },
  [
    {
      $setWindowFields: {}
    },
    {
      $set: {
        "label": {
          $switch: {
            branches: [
              {
                case: { $gt: [{ $size: "$tags" }, 3] },
                then: "AA"
              },
              {
                case: { $gt: [{ $size: "$tags" }, 1] },
                then: "A"
              }
            ],
            default: "B"
          }
        }
      }
    }
  ]
)