21 cm.

Firstly, you have to see how are you financially. In order to have kids you should have a secure place to work that can offer enough money monthly to buy the basics (which are quite expensive and differ depending if you want to have them naturally or to adopt them).

Secondly, you should try spending some time with a friend's or relative's kids and see how you get along with them.

And lastly, this being my personal opinion, you should ask yourself if you are prepared to sacrifice every aspect of your life for them. I don't want to make it sound bad, living like this is bad only for people who are not fit to be parents. Good parents always find happiness in their child's happiness and every other way of achieving it is meaningless.