The deployment map from 4th Edition Codex: Black Templars apparently added up to over 6000 Marines, but it's not very helpful since they didin't deploy at once so a bunch of them proably fought in more than one deployment. Older Codexes also mentioned 5000-6000, so did Aaron Dembski-Bowden.
On the other hand, Eternal Crusader novel mentions that 200 Marines is one fifth of total Black Templars numbers which makes them exactly Codex sized.
During Armageddon, Black Templars had about 1200 Marines, and High Marshall Helbrecht was said to have mustered three crusades. Older fluff mentioned that Black Templars only had three "primary" Crusades with multiple "secondary" ones that are much smaller, so we don't really know which they were. There are only ten Black Swords for whole Chapter so theoretically that would mean that there can be only 10 crusades at any given time, which gives us 4000 Marines when combined with Armageddon deployment number... unless maybe some "secondary" Crusades can afford not having one ready for a Champion that could arise. Also, In Blood and Fire, our most favourite Chaplain of all time mentions that they have dozens of crusades going on...
tl;dr
Nobody actually knows. Absolute minimum is slightly over 1000, reasonable maximum is around 5000-6000, retarded maximum is around 10000. It should be noted that they practically never fight all together, and most known deployments were around 500 Marines. Honestly, in my humble opinion, they are uncountable. They are way too stretched and barely keep contact with each other, nearly making crusades chapters of themselves and that proably means their numbers fluctuate a lot. At one point they might have only 1000 Marines, at other they might have 10000 Marines, but it's not like anybody actually bothers to count.
On the bright side, they will proably never truly die off due to that, so I guess the whole purpose of abusing a loophole that says a Crusading Chapter without a homeworld can have more Marines to not let them all get killed at once is achieved.