It’s hard to know where to begin to explain how wrong you are on so many counts, so I’ll just deal with the one you started this thread with. Just because an untrue story has been around for 2000 years doesn’t make it any less untrue. I am referring to the story written by Tacitus a century after the event where he describes a Roman attack on the island of Anglesey and the killing of the Druids found there. It has been assumed, with absolutely no backing, even from Tacitus himself, that every Druid in the British Isles had conveniently relocated to Anglesey so as to be there to be killed. This is clearly ridiculous for several reasons.
First, there were several tribes in Southern Britain who were friendly towards the Romans. They would not have remained friendly for long if the Romans had killed their priests, healers, law-givers and lore-preservers, i.e. their Druids.
Second, it is absurd to think that even every Druid in Wales would have made their way to Anglesey. Why would they?
Third, even in the incredibly unlikely event that every single Druid in the whole of England and Wales had been on Anglesey at the time of the Roman attack, is it really likely that they all would have stayed there to be killed when there was a regular, busy and short trade route by which they could have escaped to Ireland?
Fourth, even assuming that all the Druids in England and Wales had gone to Ireland and been too stupid to hop on a boat, the Romans never penetrated far into Scotland, nor did they invade Ireland at all. Both of those countries had Druids, unless, of course, you’re assuming that all of them also ran lemming-like to Anglesey and stuck around to be killed.
Fifth, they clearly didn’t since medieval literature refers to Druids in Ireland still being employed by local kings as late as the 10th century, while Welsh poets ares still referring to themselves as Druids in the 14th century.
6th, as Ronald Hutton and others have pointed out, Tacitus did not witness the attack on Anglesey himself, nor does he tell us his source. Tacitus himself is the only source we know of and he may have made the whole thing up.
To answer your question, then, how many Druids were there after the 1st century CE: a lot, certainly hundreds, possibly thousands, certainly thousands if you include their students.