Using the transit method, how many times must a dip in the brightness of a star be observed before it can qualify as an actual planet detection?

4 times.

Explanation:

The transit method is observed as the small drop in brightness and this occurs in the orbit of the one star that passes in front of the another star and mostly this factor depends n on the size and the duration of the planet that passes through the path. The first transit is when the smaller body is completely outside other larger bodies and is moving inward. The second contact is when the smaller body is inside and is moving further inward. The third transit is when the smaller body is inside the larger body of the planet and is moving outward. And fourth is when the smaller body is entirely outside and is moving outwards.