How would you most efficiently limit the number of items that can be included in a data store?

Hi all,

I’d like to limit the number of items a user can add to a data store to say 20, and when they hit 20 trigger an alert to let them know.

I know how I’d do this on the backend with SC, and I assume that triggering a flow or using a ternary might be the solution - but I like the speed of simply inserting, so wondered what the most efficient way to achieve the above might be?