This is an interesting, but extremely difficult question to answer. The reason for this is that the question transcends more than just a religious group. We have evidence that by 50 to 60 AD, Christianity had spread (although it was incredibly small at this time) to cities all over the Levant, ancient Greece and all the way to Rome itself. We also know, by the quality of the writing, especially in the seven letters of Paul that we know are authentic, and by the style of the Gospels, that the writers were all fairly well-educated. This is significant because it shows that By the time Paul started writing his letters (around 50-55 c.e.) until the time the Gospels started being written (70-90 ce) Christianity had gone from being a small Jewish sect of peasants to having a diverse demographical make up. (For more on this I recommend reading, Ramsay MacMullen's (a Roman Social Historian) book Christianizing the Roman Empire: A.D. 100-400 which explains this in quite a bit more detail.

That said, numbers mean a lot and Christianity needed to spread an awful lot and quickly to go from between 20-30 people (which I recently discovered is the mainstream belief for Early Christianity Historians) at around the time of Jesus' death (around 30 ce, plus/minus 5 years) to being 5% of the population by the early fourth century, which would have been 3 million people. Rodney Stark's book The Rise of Christianity although incredibly controversial and disputed by many scholars, did make one assertion that many scholars either agreed with or didn't oppose. That is, that he believed that, on average, Christianity needed to gain about 40% more followers per decade until the year 300.

So why do I mention all these numbers? Well, because it shows that A: Christianity started out incredibly small and remained small for quite some time. But B) it did get more progressively popular over the course of the first three centuries CE and that it wasn't limited to just the area around the Levant so again, we aren't just dealing with a specific group of people -- we are talking about different cultures from all over the Mediterranean, most of which had different practices for bearing children.

However, it's safe to say a few things demographics wise. On average, typical Jewish peasant who lived an agrarian type of life-style could typically have up to 4-8 children, of course not all would live to be adults. Those who lived in cities or were more wealthy probably had fewer children, but again, we have no exact numbers on this. I am not qualified to answer how many children people in Ancient Greece or Rome would have had, so I digress from answering that aspect of the question.

TLDR: Ancient Hebrew peasant families typically had 4-8 children, yet Christianity had spread to multiple cities and states over those first few decades, so individual studies would be needed to answer the question in more detail.

Sorry if this wasn't the exact answer you were hoping for.