It depends what you mean by that. By “coming in,” many mothers think that means they should be painfully engorged. However, if a baby starts breastfeeding really well, mothers should not get engorged. Yes, they get “full” around three or four days after birth, sometimes later, but they don’t get painfully engorged. This sort of engorgement is usually due to a number of factors, including the baby not breastfeeding well and the mother getting a lot of fluids during the labor and birth.
Some mothers are not physically capable of producing enough milk. But they should breastfeed anyway. By supplementing using a lactation aid at the breast, they can breastfeed as long as they wish.
Many mothers who “don’t have enough milk” actually could have but they got off to a poor start. How a baby latches on can make a huge difference in how well she gets milk from the breast, for example.