If you only have the one cardinal, your chances of having baby cardinals are 0. The eggs need to be fertilized by a male after the female has laid them. No male, no fry. If you have a male, you would have seen him swim right next to her as she laid the eggs and release his sperm. If you did not see this, also, very unlikely they were fertilized. Lighting, most Tetra eggs are photosensitive so if you have any direct light over them, they tend to not survive. If you try to deliberately breed Tetras, you want to do it with indirect lighting. ( I kept my tanks near a window but not directly next to it so I could have some indirect light. The fish would spawn at dawn.) Accidental spawnings happen but huge numbers of surviving fry from this type of breeding are rare. You tend to get just a few survivors if you are lucky. Considering they laid up to 100+ eggs, a few survivors is not a good ratio.
Hope this helps