You can create any number of cards to any column.
click on the add card button, fill it out (this is a ck editor instance, it supports rich text and IPS features such as @mention). There is no limit to how much text can be written here, but keep in mind its meant to be a "title" area for the card. To save a card, just click outside of the card and it will auto save.
You can assign a card to up to 10 people (default is 6, but you can change this in project settings). You can also claim the card for yourself.
you can move cards vertically in a column to reorder them, or drag and drop them to another column.
if you click on a card, it will open up in a popup, giving you access to various things. you can add a description for the card, add a list, mark a card complete, set a due date, view the history of a card and/or add/read comments.
there is no way to "delete" a card, but you can archive it. grab ahold of a card and drag it to above where the columns display, the archive area will open up, allowing you to drop the card there. you can retrieve a archived card at any time.