You can write names, surnames, phone numbers, the full name or a combination of all of them. In fact, you can add whatever you want and however you want. However, it is important to look at the preview box to make sure that the text is not too long and legible, which can reduce the font size. The best font for texts with many characters is number 1, which gives more visibility to the text.
It is also important to keep in mind that the Packs are made up of labels with a single line and others with two lines, so the text on the second line must be complementary since it is only visible on large labels that do allow 2 lines of text.