countDocuments()

countDocuments(filter) runs a real count of matching documents. Use it when the number has to be right — '4,312 completed orders'. The old count() helper is deprecated; do not write it in new code.

db.users.countDocuments({ city: "Bangalore", isActive: true })

2

db.orders.estimatedDocumentCount()

estimatedDocumentCount() reads collection metadata. It does not take a filter. It can lag slightly on a busy replica. Use it for 'roughly how big is this collection?', not for a badge that says 12 results. countDocuments({}) on a huge collection is a scan (or a fast count if the planner can use an index) — do not put it on every list endpoint next to skip. Offset pagination that also countDocuments the same filter doubles the work.