I had six tanks with my first bc I preferred them over bras. I also had a summer baby so I would wear them around the house. I had two nursing shirts that I hardly wore bc they were annoying to have ly flat.
I'm getting more nursing tops this time. I tried the tank and cardigan combo with DD, but I had to wear nipple pads 24/7 and they were pretty obvious. I'm also in a more prominent position for my job now so I need to look a little more polished than just the tank look all the time.
I also never liked wearing sleep bras because I really needed the real bra to keep everything in place overnight. So basically, this time I'm doing:
3 nursing tanks 3 nursing bras and I guess 5+ nursing tops. Then I'll have other tops where I'll just lift up the top layer and maybe DYI some nursing belly covering tanks out of a few cheap forever 21 camis. Like this: www.babybellykelli.com/diy-nursing-tankcami-in-under-10-mins/#more-3877
I have five nursing tanks/camis right now. I started with one and have added four in the last two weeks. Mostly because they are the easiest thing to sleep in and wear under sweaters. I have a few non nursing sports bras that work at night too. I have three nursing bras, and 4 nursing tops. Like I said though I prefer nursing camis under normal clothes.
You will figure out what you like and can add later. As long as you have something that will work right away, you should be fine.