Dermatologist gave us ivermectin & permethrin for scabies. Do we need to use the permethrin twice, or was the one time enough?

Depends on symptoms. Was the ivermectin oral or topical (lotion)? Did they tell you to repeat that dose in 14 days? Some doctors recommend automatically repeating the permethrin in 14 days; others advise on repeating only if there are live mites still present or symptoms persist. You should ask your dermatologist.