Multithreaded Server C++, Socket Programming

I am learning socket programming. I have built a client server program, where each client is handled by an individual thread in server program. But after a certain number of threads the system's performance will degrade. Now, to solve this, my task is to make my server handle multiple clients (say N connections) with a single thread, and only create a thread when there are more than N connections, and manage those N connections.

Say, 1 thread currently can deal with 200 clients, I will create another thread, when 201th connection arrives.

How to achieve this? And what is the proper way to manage those N connections?

Here is my code:

Client Code

#define PORT 9876

bool client(struct sockaddr_in* server_addr, int& socketDesc)
{
    memset(server_addr, (char)0, sizeof(sockaddr_in));
    
    sd = socket(AF_INET, SOCK_STREAM, 0);

    if (socketDesc < 0)
    {
        cerr << "Cannot open socket" << endl;
        return false;
    }

    server_addr->sin_family = AF_INET;
    server_addr->sin_addr.s_addr = inet_addr("127.0.0.1");
    server_addr->sin_port = htons(PORT);

    int n;
    if ((n = connect(socketDesc, (struct sockaddr*)server_addr, sizeof(sockaddr_in)) < 0))
    {
        cerr << "Connect Failed" << endl;
        return false;
    }

    return success;
}

int main(int argc, char* argv[])
{
    
    int socketDesc;
    struct sockaddr_in server_addr;
    
    client(&server_addr, socketDesc) > 0)
    
    const char* hello = "Hello from Client Side";
    

    for (int i = 0; i < 10; i++)
    {
        cout << hello << endl;
        send(sd, hello, strlen(hello), 0);
    }

    return 0;

}

Server Code:

#define PORT 9876

void server(struct sockaddr_in* server_addr, int& socketDesc)
{
    memset(server_addr, (char)0, sizeof(sockaddr_in));
    
    socketDesc = socket(AF_INET, SOCK_STREAM, 0);

    if (socketDesc < 0)
    {
        cerr << "Cannot open socket" << endl;
        return;
    }
    server_addr->sin_family = AF_INET;
    server_addr->sin_addr.s_addr = INADDR_ANY;
    server_addr->sin_port = htons(PORT);
    int n;

    if ((n = bind(socketDesc, (struct sockaddr*)server_addr, sizeof(sockaddr_in))) < 0)
    {
        cerr << "Cannot Bind" << endl;
        return;
    }

    if ((n = listen(socketDesc, SOMAXCONN) < 0))
    {
        cerr << "Cannot Listen!" << endl;
        return;
    }
}
void func(struct sockaddr_in* client_addr, int& new_sd)
{
    char memory[1024] = { 0 };
    int n;

    while ((n = recv(new_sd, memory, 1024, 0)))
    {
        cout << "Current Thread ID " << this_thread::get_id() << endl;
        cout << buffer << endl;
    }
    
}
    
int main(int argc, char* argv[])
{
    int socketDesc;
    struct sockaddr_in server_addr;
    server(&server_addr, socketDesc);

    vector<std::thread> poolofthread;
    struct sockaddr_in client_addr;
    socklen_t length = sizeof(sockaddr_in);
    int new_sd;

            
    while ((new_sd = accept(socketDesc, (sockaddr*)&client_addr, (socklen_t*)&length)) > 0)
    {
        cout << "Client Accepted" << endl;
        thread t(func, &client_addr, ref(new_sd));
        poolofthread.push_back(move(t));
    }
    return 0;

}

This Code for each client handled by separate thread is working fine.

I have tried handling each client with a single thread. Now, I want to handle N connections with a single thread. And only create a new thread when it is necessary(LIKE, when more than N connection arrives).

If you can give some idea about the best way to manage those N connections, then it would be helpful. I am learning select(), poll and epoll. How can I use them and which will be best with my case and why?

I have tried clearing the question and the problem I am facing. If you don't understand any part please write in the comments.