As Arsenal Thresher enters the battlefield, you may reveal any number of other artifact cards from your hand. Arsenal Thresher enters the battlefield with a +1/+1 counter on it for each card revealed this way.
If multiple Arsenal Threshers are entering the battlefield at the same time (for example, if Warp World is resolving), you may reveal the same artifact cards from your hand for each of them.
If Arsenal Thresher and another artifact are entering the battlefield from your hand at the same time, you may reveal that other artifact for Arsenal Thresher. If that artifact is a second Arsenal Thresher, you may reveal each Arsenal Thresher for the other.