It is recommended that if you want to have a baby, it is better to have two and the preferable age difference is 20 -30 months. Is there the series of methods to create a happy and successful life for our only child?

Under a dozen. I am sure many old children might argue that they turned out pretty good so it isnt the same for everyone. Having 5 children of my own i see how they had to learn to life together, help each other, be patient with the ones who required that and learned to serve in a family. That was pretty great too. Each person has to decide that issue for themselves. You can wait and see if you still want anothe.