Can I set WIP limits in a board?

Yes, you can set WIP (Work-In-Progress) limits for each column in a board. WIP limits help you control the flow of work through the board, by limiting the amount of work that can be in a column at any given time. If there is more work than this, a warning will be shown:

WIP limit exceed in a board column

To configure WIP limits for a column, click "Edit this board" at the bottom of the page. Then open the popup dialog for the column:

Setting WIP limits in a board column

You can set limits based on number of cards, or amount of work remaining, or both. And you can set limits on the total for the column, or per person, or for the unassigned work in the column.