There is simply no way that we can give anything more than a vague estimate of Christian numbers by the end of the 1C AD. At least one ancient document in our possession tells of the growth in numbers of Christians, and this was written some years before the end of the century by Pliny the Younger the Governor of the Roman province of Bithynia to Emperor Trajan.
"Many of every age and social class and even of both sexes are being called to trial. Not just the cities but villages and even rural areas have been invaded by the infection of their superstition" [Epistulae 10.96] Although he does not report any numbers, his reference to the presence of Christians throughout the Province implies a recognizable status given to this persecuted minority. It must also be remembered that Bithynia was one of the more remote provinces of Empire, and is therefore suggestive of growth in even greater numbers elsewhere. Although as the NT shows Christianity started as an urban phenomenon, it soon grew by increments and missionary activity to the countryside and involved all sections of society.
Dr Samuele Bacchiocchi writing in the Andrews University historical site has estimated that by the end of the 1C AD the population of Christians had grown to about 1% of the population of the Empire, which was in the region of 100M-180M. This would place the numbers of Christians living then to about 1M. The World Christian Encyclopedia [published in 1982] confirmed this number by suggesting a more precise percentage of .6% of the Empire's population.
The phenomenal growth of Christianity, which as we have seen has been attested to by at least one contemporary historian of the 1C AD, can probably be attributed to two factors, one of which is not acknowledged by the academic community. The first of these was, of course persecution, including the one by Domitian toward the end of the C AD which, incidentally, caused John's exile to Patmos. The other is the role of the Holy Spirit whose Divine Providence oversaw the growth of believers in Christ.
By Acts chap 3 the numbers of believers had grown from 120 to 3000 [what is that? 450% increase?] Then, just one chapter later, in Acts 4:4 we are told of a further 5000 men who were baptised. We would need to factor in at least the same number of women who are not named, making some extra 10 000 in this short time. This means that within the passage of days of the founding of the Christian community we are having to deal with at least 13 000 people who became believers. To suggest, as the Watchtower suggests that somehow this miraculous growth ceased at this time and numbers were more prosaic, is to misread the authority of the Holy Spirit. And it is to manipulate not just the miraculous in the early years of Christianity, which the Watchtower supposedly recognizes, but is also a deliberate attempt at fiddling with figures merely to prop up a doctrinal aberration.
According to the Watchtower's own ignorant theology an "apostasy" developed at the turn of the first century leading to countless people having given their lives in vain during monstrous persecutions of extreme brutality. Only an estimated 10% of those torn asunder by wild animals and gladiators is suggested to be a true number of real believers. This is but a barbaric reaction to the memory of people whose blood has stained the pages of history, and reflects the same monstrous attitude that was manifest by the bloodthirsty spectators who had no sensitivity to the death of others. Whatever the Cause, giving one's life for it makes one a True Believer.
Countless Muslims have been killed by Hindus, and countless more Hindus have been slain by countless other Muslims all in the name of Belief. Whether we acknowledge the verity of such belief or not, acknowledgment must be made at the absolute commitment of belief, of those who permit their blood to be poured out. No amount of idiotic gainsaying, in the name of dogmatic bigotry can erase that depth of commitment.
For the Watchtower, the figures simply do not add up.