Hey hey cardinal tetras thrive in low
pH 6.0 to to 6.5 or even lower if you want them to breed. They also like very soft water, and thrive in a heavily planted tank. They will get about 2 inches in lenght, so you could go with 10 to 12 of them. I would suggest adding some shrimp like amano etc, to do algae work. In a well planted tank you could add about 6 amano with the cardinals and should be fine for bio load.
Also cardinals can be very sensitive to change, so you want to very slowly let them into the water, take at least a hour of floating and every 10 min or so let a bit of your water into the bag until the bag is a good mix of your water and the bags water. I would add 6 and then add 6 a couple weeks later and not all 12 at once.
Also they need a mature tank, if your tank is still new, I would plant it very heavy (this will take some of the load of
ammonia and nitrites etc off your tank) And add the shrimp once all your levels are good 0 ammonia 0
nitrite and 10 to 20
nitrate. Then make sure everything is stable for a few weeks, then start the phases of Cardinals (granted a few weeks after cycle is not really mature, but the plants will aid in the mature health of the tank). If you try to add cardinals to a non mature tank there is a large chance they will die. Once your tank is mature and running strong Cardinals are quite hardy, but they will non last long if you try and put them in a tank that is non ready for them.
Some people say Cardinals will adapt to higher ph and harder water, but the chances of loss are much higher in those conditions. They also tend to like water on the warmer side, 78 to 81 degrees, I have yet to loose a single cardinal in my tanks that are low ph soft water and in that temp range other then to old age.
They are beautiful fish and look amazing in a planted tank, and really need good numbers to stay happy, its a great choice to go the route your going and stick with a Cardinal tank rather then a few of them and a few other fish etc.