How to get total document count in aggregation pipeline?

db.getCollection('permission').aggregate([
   { $match: { "accountId": "my-account" } },
   { $group: { _id: "$accountId", "totalCount": { $sum: 1 } } }
])

db.getCollection('permission').aggregate([
   { $group: { _id: null, "totalCount": { $sum: 1 } } }
])