Step 1: Definition. A joint family is a family structure where extended family members live together under one roof.
Step 2: Indian context. Uttar Pradesh, being the most populous state, has the maximum number of households. Joint family traditions are still common, especially in rural UP.
Step 3: Conclude. Thus, Uttar Pradesh has the highest number of joint families.
\[
\boxed{\text{Uttar Pradesh}}
\]