The general rule of thumb for how many bras you need is one in the wash, one to wear and one in the drawer. Same goes for nursing bras! Some nursing moms also feel more comfortable wearing a sleep or leisure bra at night especially in the early months. The other thing to consider is how often you do laundry. If you do laundry daily (like in my house!) then 3 or 4 nursing bras will be plenty. If you do laundry less often, then 6 or 7 may be more sensible for you.
Look into purchasing your first nursing bra towards the last trimester so that it will accommodate your new fuller breasts. Choose your nursing bra based on how it fits you and one that provides easy access to the breast for feeding. Check out these fit tips for more information.
Machine washing & drying tends to stretch out the bra and damage the elastic. Also the hooks and latches could break – so it is recommended that you hand wash and air dry your bras to get maximum wear out of them.