How does having all of the planets in your twelfth house be in retrograde on your birth chart effect you?

The twelve house deals with introspection and intuition. Retrograde planets tend to express themselves in a more internal manner than direct ones. A native with a 12th house with more than 3 planets all retrograde will likely have a very shy, retiring nature. They are likely to be very imaginative and intuitive, with a love of private time. They may not be particularly forthcoming with things the planets occupying this house govern or relate to in the chart.