My wife has been breastfeeding our baby boy since birth. And milk demand has naturally increased over time as baby grows. How can she increase her milk supply or should we go for a mixture of milk powder feeding and breast milk feeding? Any advice or recommendation?

There is no need to have a mixture of formula if the supply sufficient for baby. Breastfeeding is about demand and supply. Try to let baby latch more to maintain or increase supply. Alternatively, she can go for power pump. What works as a booster for me may not work for your wife. Everyone is different. Oatmeal is a common booster for many breastfeeding mothers. She can take with frisomum or any type of milk.