You need to have a sufficient number of incomings for any information to be possibly extracted
Honestly, depending on the timings of them, but if a person sends 10 attacks from different villages (or at different speeds), around the same time frame, and you can guess at them with almost 99% accuracy.
and you need to have a certain number of them already timed.
You don't actually need any of them already timed, as long as enough are sent to find the pattern. Enough could be a few attacks if they are long range, or more attacks if they are shorter range, but you'll be able to flip through, and figure out the pattern of time sent, and back tag them, beyond a shadow of a doubt.
Not sure if this pic helps with this guide or not, we have a much simpler guide in our forums for back tagging, I pulled this from there to show the time pattern your looking at.
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These 8 attacks, you could identify without a doubt at any point in their travel to the destination, without ever knowing a single one prior.
In fact these ones were 33-34 hours old when I tagged them.