Maximum number of docks

Hi,
I have an instance with a limited number of docks (=d) at the depot (site). This means at most d vehicles can be serviced simultaneously at the location. If there are more, e.g. returning from the previous leg of the route, going to the next one, they have to wait for an empty dock to be loaded (serviced).
What is the most elegant way to model this?
Thank you!