Why did Romans name their children by numbers?

the roman naming system was tripplefold:

praenomen (First name) was chosen by the pater familas at the ninth day of a childrens life. Classic romans used only very few different names. (I think alltogether we know about 20-30 names). These were used by most families, so there were really many people named Lucius. Some names were only used by one family

Then one had a family name: nomen gentile. (Iulius, Antonius,...)

And a chosen name. Cicero,... Many of these names were funny or even insulting. Names like "fat man" or "drunkard" or "burnt one" are known to us.

To answer the original question: Why did romans pick numbers. For convenience sake most likely. People had many children and it was easy just to number them. Why is there no numer1-4 and 11/12: Because there were enough names to name a few children, but to little no name all. And I do not think that a woman lived to bear more than 11/12 children. In fact: many upper class roman woman did not wanted to have more then one male child, because they feared for their waist (One of the old writers complains about that)