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

You can: I usually have patients use it twice. One week apart.

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.