A valid trendline needs at least three touches: two define the line, and the third confirms its validity as genuine support or resistance. Two points define the line, and the third confirms it.
Two touches alone is just geometry; any random pair of swings can be connected. The third touch is the confirmation that solidifies the trendline; after three touches, the market has proven it respects the line, and that third touch is often the entry point.
More touches strengthen the level, but expect a breakout near the fourth or fifth test as participation thins.