Create N subscribers to N topics without knowing value of N

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I think you are asking something that makes use of #q63991 , the following example creates n subscribers and passes on information to the callback to be able to distinguish them:

#include "ros/ros.h"
#include "std_msgs/String.h"

void chatterCallback2(const std_msgs::String::ConstPtr& msg, const std::string topic)
{
  ROS_INFO_STREAM(topic << ": I heard: " << msg->data);
}

int main(int argc, char **argv)
{
  ros::init(argc, argv, "listener");
  ros::NodeHandle n;

  std::vector<ros::Subscriber> subs;

  int num = 10;
  ros::param::get("~num", num);
  for (size_t i = 0; i < num; ++i)
  {
    std::stringstream ss;
    ss << "topic_" << i;
    const std::string topic = ss.str();
    ros::Subscriber sub = n.subscribe<std_msgs::String>(topic, 10,
        boost::bind(chatterCallback2, _1, topic));
    subs.push_back(sub);
  }
  ros::spin();

  return 0;
}

A typical pattern a bit more complex than this would be to have a std::map using the topic names as keys and data in the callback is stored or manipulate using that map.