How to generate a random number with each instance of class initialization and creating a vector

I am trying to create a new instance of a class each time the function iterates and render it onto the screen. Unfortunately, the second times the code iterates the rand() function gives back the same xPos and yPos integer. I tried to put srand(time(NULL)) in the constructor but I have read its a bad habit to call srand() function multiple times. Does anybody know what causes this behaviour?

Main.cpp

#include <iostream>
#include <SDL.h>
#include <vector>
#include <time.h>
#include "WindowAndRenderer.h"
#include "Entity.h"

int main(int argc, char* args[])
{
srand(time(NULL));
Entity animals;

std::vector <Entity> boids;

for (int i = 0; i < 5; i++)
{
    Entity animal;
    boids.push_back(animal);
}

WindowRenderer WindowRenderer("natural selection v1.2", 800, 600);

WindowRenderer.loadMedia("include/arrow.bmp");

WindowRenderer.Loop(boids, animals);
WindowRenderer.~WindowRenderer();
return 0;
}

Entity.cpp

#include "Entity.h"

Entity::Entity()
: xPos(rand() % 800), yPos(rand() % 600)
{
dimensions = {xPos, yPos, 8, 8};
}

Entity::~Entity()
{

}

int Entity::getx()
{
return xPos;
}

int Entity::gety()
{
return yPos;
}

SDL_Rect Entity::getDimensions()
{
return dimensions;
}

Loop part in WindowAndRenderer.cpp

while (isRunning)
{
    while (SDL_PollEvent(&event) != 0)
    {
        if (event.type == SDL_QUIT || event.key.keysym.sym == SDLK_ESCAPE)
        {
            isRunning = false;
        }
    }
    // this is where everything gets updated
    SDL_SetRenderDrawColor(renderer, 255, 255, 255, 255);
    for (int i = 0; i < boids.size(); i++)
    {
        SDL_RenderCopy(renderer, tex, NULL, &boids[i].getDimensions());
        std::cout << boids[i].getx() << std::endl;
    }
    SDL_RenderPresent(renderer);
    boids.push_back(a);
}