If there is an official ban for sailing with pleasure boats in the Netherlands in connection with the Covid19 pandemic and marinas are closed, you can postpone your sailing holiday to another period of this season or possibly the next season free of charge. If the prices are the same for this period, you don't have to pay anything extra. If the prices are higher, you will be charged the difference. As long as the lessor can provide the service and there is no rental ban, there is no right for free cancellation and the normal rental conditions apply. If you are not allowed to travel abroad by the government of your country, this is your risk and the cancellation policy stated in the rental conditions do apply.
For the permitted number of crew members on board, you must adhere to the guidelines of the Dutch R.I.V.M.
An interim change to the guidelines, allowing fewer or more crew members on board, does not entitle you to a free cancellation or rescheduling of the rental period.