std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::bucket_size - cppreference.com

From cppreference.com

size_typebucket_size(size_typen)const;(since C++11)Returns the number of elements in the bucket with index n.

Parameters

n - the index of the bucket to examine Return value

The number of elements in the bucket n.

Complexity

Linear in the size of the bucket n.