How many spears would you have to send at an empty player for none of them to die? and how many would you have to send at a barbarian village for none of them to die.
I can't remember but i think it was 9, or was that with a paladin?
Assuming wall 0:
you'll only ever lose 1 spear at most unless you get unlucky with an attack anything over 1 spear, from what I recall. The only reason not to send with that few is because the loss of 1 spear = 50/30/10 => 90 haul, and the surviving spears would have to haul more than 90 in order to make it profitable.
Thus, it doesn't become profitable to farm until you have 5 spears (1 dies, 4 survive, 4x25 = 100) but the gains are, as you can see, minimal, and usually not worth it.
at 17 spears, you can have no losses IF you get maximum luck (25%).
at 33 spears, you will have no losses with non-negative luck (i.e. 0% or greater).
at 76 spears (or thereabouts, I think) you will have no losses no matter what luck you get.
Sent with a paladin, you will never lose even a single spear unless there is a wall or defence, afaik.
For wall > 0, all these values will obviously differ, and will (again, obviously) need to be much higher to counter the defence bonus of the wall.