The total number of plants needed for 12 dinner salads (without loss to insects or rabbits) is 12 / 3 salads from each lettuce plant = <<12/3=4>>4 lettuce plants
Since Anna expects to lose half of her plants to insects and rabbits, so we need to plant twice as many plants or 4 * 2 = <<4*2=8>>8 lettuce plants
#### 8