Hi there, you may be laboring under some misapprehensions. "Biblical times," if by that you mean the times described in the modern Bible, did not have a Christian church, although the gospels and other parts of the New Testament were written during the early days of the church. Could you maybe describe your question in more detail and we'll see if we can help you phrase it for a better answer?

I just answered a variant of this question a few days ago, so I copied and pasted the answer here and added more to the ending to answer your second question.

Question 1 This is truly an excellent question. One that could actually take pages, chapters and even books to answer (which many have done and I will cite below) but I will do my best to try and help answer it briefly.

Right off the back, it can be safely argued that Jesus was not relatively popular around the time of his death (which we can safely argue was probably around the year 30, plus/minus 5 years) however, he would have had to have had at least a somewhat sizeable number of people for his teachings to gain followers. Why do we know that? Well, there are a few things that make us think this way. First, the first person to start writing about Jesus was Paul, who started writing sometime around the year 50. Now it’s important to recognize that Paul never met Jesus while he was alive, so he never wrote about many things Jesus did or said (this has no bearing on the theological ramifications around Paul's writings). Historian Bart Ehrman actually wrote about this in his book last year: How Jesus Became God: The Exaltation of a Jewish Preacher from Galilee Where in it, Ehrman explains:

The problems with Paul are that he didn’t actually know Jesus personally and that he doesn’t tell us very much about Jesus’s teachings, activities, or experiences. I sometimes give my students an assignment to read through all of Paul’s writings and list everything Paul indicates Jesus said and did. My students are surprised to find that they don’t even need a three-by-five card to list them. — (Page 88).

It is important to note that it is widely believed that other Christians had started writing about Jesus prior to this period, but that their work no longer exists.

So why do I mention this about Paul? Well, it’s because it shows that Jesus’ followers had started to increase by this time period (20 years after his death). It also shows that by the time Paul started writing, there had to have been a sizable and somewhat diverse group of people who started following Jesus’ message. The writings of Paul are quite well-written (for their time) and the Gospels (which would be written in the following few decades) are even more professionally written for their time (Mark being the least well-written and Luke being the best). This does suggest that in order to get a sizable amount of well-educated people to write down these stories, regardless of their errors, there had to have been a decent size following or their population would have faded away (like many other cults during this time period.)

I will say though, that his followings couldn’t have been too large. I argue this because there isn't a whole lot about Jesus written by non-Jewish/non-Christians (I use this last term loosely when applying it to the first century CE) prior to the second century. As some people may note, the first person to write about Jesus and his followers who was not a Jesus follower was the Jewish historian Josephus, who would only briefly mention Jesus in his twenty plus volume history of the Jewish people called The Antiquities of the Jews. Here, Josephus discussed the execution of a man named James, whom Josephus described as “the brother of Jesus who is called the messiah” (Ant. 20.9.1) (I’d also like to note that Josephus never believed in Jesus being the Messiah, he was merely stating what others had said). He does also discuss Jesus again, but I won’t bore you with ancient quotes that would take me far too long put into context.

Long story short: Followers of Jesus would have had to have had a decent number behind them by the time Josephus left the Levant (area near present day Israel) in order for him to have mentioned them at all, but due to the fact that he barely says more than a paragraph about him in all his writings suggest that Christianity (as it would soon after be called) was not too popular, and by implication, the Christian Churches would not have been fairly large.

TL;DR: It’s likely that Jesus was not too well-known at the time of his death (the average Jewish peasant was unlikely to have heard of him had they not lived un Judea), but his following would have had to have been sizeable enough to spread the stories of his life by oral tradition in order to gain such a noticeable following by the second half of the first century CE. It’s also worth reading John Dominic Crossan’s book: The Historical Jesus: The Life of a Mediterranean Jewish Peasant to gain more information about this very topic.

Another great book that actually deals with the very people we are talking about (early Christians in the immediate aftermath of Jesus' death) was done by an excellent historian named John Dominic Crossan, who wrote a book called: The Birth of Christianity : Discovering What Happened in the Years Immediately After the Execution of Jesus . This is definitely worth a read for anyone who is interested in this topic. I'm ashamed I didn't remember it earlier.

Question 2 Now if you are asking about about size of the Christian Churches through the first few centuries, well that's an equally interesting question. One thing I'd like to point out is that Christianity didn't spread rapidly until the fourth century when Constantine issued the Edict of Milan around 313, making Christianity legal. (It should also be noted that the common notion that Christians were widely executed and punished for their beliefs are bit exaggerated. Candida Moss's book The Myth of Persecution: How Early Christians Invented a Story of Martyrdom outlines that although illegal for this period, Christians were generally able to worship without constant threat of retribution). However, it still can be safe to say that no one was really embracing of Christianity until after Constantine legalized it.

Now the actual number of Christians in the Empire is still up for debate. In his book Lost Christianities: The Battles for Scripture and the Faiths We Never Knew, Bart Ehrman (and many other Early Christian History Scholars) estimate that maybe around 5% of the population was Christian prior to that part. Regardless of the accuracy of that number, it's safe to say that Christianity boomed during the fourth century. By the end of the 4th century, Rome, which was primarily two separate empires now, were both heavily populated and dominated by Christians.

Later Edits After thinking about this question, I dug a little more into a book I had to read for a class last year that actually had an exact number, which I know you were hoping for. Rodney Stark, a sociologist, wrote a book called The Rise of Christianity: How the Obscure, Marginal Jesus Movement Became the Dominant Religious Force in the Western World in a Few Centuries and discuss this very topic. Now I want to explain that Stark is a sociologist, not a historian and this book was actually attacked for several reasons by many scholars. However, he did make one point that was actually agreed upon by other historians. The first is that Christianity, as I said above, started out quite small, possible as little as 20-30 people by the time that Jesus was executed. However, it can be reasonably assumed that a steady growth rate of about a 40% increase to these followers happened (on average) every decade until the fourth century, where it exploded even faster.

edits for spelling

Also, you say "the Christian Church" as though there was only one. There were several branches of Christianity from as early as immediately following Christ's death, and especially after the Judea uprisings in the 60s CE (at least according to the BBC documentary miniseries, 'A History of Christianity' hosted by Diarmaid MacCulloch)

I actually did a little more digging into books I had to study last year and found some actual numbers for you. I added it to an "edited" part of my answer at the bottom. Hope it helps