It can be hard to know how many breastfeeding tops to buy initially. To really come to an answer you first need to decide how often you are likely to be breastfeeding in public, and how comfortable you would be in non-maternity clothes while breastfeeding.
The factors you should consider when buying a breastfeeding top include:
The best breastfeeding tops for nursing in public are the ones which your breast is accessed from below. This means there will be fabric over the top of your breast throughout the feed. This makes it a lot less obvious that you are breastfeeding to a casual passer by.
One thing that can be annoying with this type of breastfeeding top is that the material sitting above your nipple can slip down during the feed, which can make it difficult for your newborn to stay attached, or in some cases breathe. Look for a top that has a small amount of of elastic to keep it up, but not so much that it squashes your breast (a recipe for blocked ducts and mastitis).
The only way non maternity clothes can accomplish this is if they are pulled to expose your abdomen and this can be uncomfortable for two reasons. One your skin may not be something you wish to show off at this time. Two - it can get a bit cold having your abdomen and sometimes some of your back exposed for the time it takes to breastfeed a newborn.
Some breastfeeding tops are accessed from the top in the same way that a bra opens up. These are very convenient and will often have a built in bra in them, meaning you don't need to buy as many bras, but you will be more obviously breastfeeding. To make yourself less conspicuous if you are worried about this you can wear a shirt or jacket over the top of your top.
Some tops are accessed through a slit, where the material pulls to the side. If you would like to buy a top like this it can be very useful to try it on and see what it's like to get your nipple through the slit. Sometimes these slits are not well placed for all mothers. Some women's breasts "point" straight ahead and some out to the side. If the slits are too far across to comfortably get your nipple through (and to be comfortable it means you should not need to pull the breast sideways to get it into position) then this breastfeeding top is not the right one for you.
On final tip to consider when choosing your breastfeeding tops is that in the early days you are likely to have a very easy let down reflex. Hearing a baby cry (not only your own) can cause it. Some women with particularly sensitive let downs can even have them if they suddenly have a warm breeze move past their breasts!
This being the case you will either want a top which is dark in colour or with ruffles to help hide the leaks, or one which has material that is thick enough to hide the rumple of a poorly placed breast pad.