T.B.
I would strongly suggest you do NOT buy a nursing bra yet. You will have wasted your money. For some women, like you and for me (I'm 34 weeks with my 4th baby), our breasts get large during pregnancy. After my milk comes in, my breasts get mammoth sized. When I had my first baby, I made a mistake by buying a nursing bra a week after I had my baby. My breasts were huge for 2 months...and the bra I bought was too small. Waste of money. What I did with my other pregnancies was buy large and x-large sports bras that were not tightly fitted. I wore those until my milk supply leveled out...and then I bought a nursing bra. You will likely get larger after the baby is born but you will not likely stay that way the entire time you breastfeed.
As for how much to expect to "grow" after your milk comes in, Elena, every woman is different. I'm huge now at 34 weeks. I know, from experience, I will get larger after the birth of my baby so I will wait until my milk levels out to the demands of my baby before buying a nursing bra.