how can I make a list glow in yellow when more than one card is added to the list?

[Tape 6ixteen]

how can I make a list glow yellow when more than one card is added to the list?

I want a list to control how many card can be moved or added or created into it at anytime. for example I want the list to contain only one card at a time and in order to add a new card to this list one has to move or remove the current card in the list. and If possible I want the title of list show as 1/1 meaning it can contain only one card at any given time. please help me figure out how to create such function inside trello for agency.

[Christl_Email for Trello]

Hi @Tape 6ixteen 👋

Christl here from the Email for Trello Power-Up. Welcome to the community!

Have a look at the List limits power-up.  You can set a card limit on your lists and if more than the limit is added, the list will be highlighted in yellow.

Hope that helps!