multimap_name.size()

// C++函数示例
// multimap::size()函数
#include <bits/stdc++.h>
using namespace std;
 
int main()
{
 
    // 初始化容器
    multimap<int, int> mp;
 
    // 以随机顺序插入元素
    mp.insert({2, 30});
    mp.insert({1, 40});
    mp.insert({2, 60});
    mp.insert({2, 20});
    mp.insert({1, 50});
    mp.insert({4, 50});
 
    cout << "multimap mp has " << mp.size()
        << " number of elements";
    return 0;
}

multimap mp has 6 number of elements