I've spotted a few questions about the Borg and the number of ships in Starfleet. This led me to think about what the best estimate of the size of the Borg collective is at any time in the standard time line? I realise that it's probably mind-bogglingly large, but still so is space.
A good answer could probably be broken down into a few snapshots across time, e.g. when they are first encountered, when Voyager finds them in the Delta Quadrant, etc.
Any answer should stick to the standard timeline, but cookies may be rewarded for including alternate timelines.