If you want your team to only be starting work on tasks, that they can finish within a given time, a good way to do it is by use of the Estimated Time field, and limiting the Work in Progress by these estimated values.
There is a specific WIP limit setting available for this purpose - it limits the number of tasks in the Working column by the amount of time, that has been estimated for these tasks.
So, if you have an 8-hours workday, and tasks on your boards have been estimated time-wise with the Estimated Time field, you can apply this setting to make sure you’re only getting started on as many tasks as you will be able to handle in these 8 hours.
To configure it, please make sure that you have the Estimated Time field enabled for your cards, this can be done from the board’s Settings - Card Template.
After that, simply go to your Board Editor and set the WIP limit to the needed number of estimated hours per lane.
You can learn more about the benefits of using the Estimated Time field here.