Install Kanban WIP for Trello via the Chrome Web Store.
A work-in-progress limit can be added to a Trello list by adding the limit to the list title in braces. If you name your list "QA ", this sets a work-in-progress limit of 5 items to the QA list. When the number of cards in the QA list exceeds 5, the list will be highlighted in red.
Updates for performance and reliability
Issues, feature requests, bug fixes? This project is on Github at https://github.com/NateHark/TrelloWIPLimits