According to the documentation on Configuring Columns - Setting Column Constraints:
Constraints specify how many issues a column can contain. You can specify constraints for all columns, or just some of them.
You can choose to exclude sub-tasks from the count. This is useful if you want to constrain the number of major tasks (stories) your team is working on, rather than the total number of individual tasks. This is likely to be more relevant to Scrum teams than to Kanban teams.
Reading the docs, I believe the column should display only the maximum number set in the column constraint, but it doesn't (tested for both Scrum and Kanban). The only difference will be that column header now had a red line added: