By default the widget show 1 passenger in the booking widgets. It is always 1, by the default pricing category. Just like shown below.
However, in those instances where it doesn´t apply to choose by default 1 in the default pricing category, this can be changed and 0 can be made appear in all categories.
To enter the widget start by selecting the "Cog wheel in top right corner" then you select “Widget” from the side bar:
Select the Time selector widget you intend to edit from the list. Note, this setting is only for the Time Selector Widget, not the checkout, car rental or accommodation widgets.
Almost at the top of the page there is a check box "Must have at least 1 participant in default price category". Un-check this box to show 0 in all pricing categories. Then scroll to the bottom of this page and save. Then your widget will update automatically.