Adding a field collection item to a form

Hi,

I've been trying since about 2 days to add a field collection item to a form, programmatically. Basically:

- I have a content type with a field named field_passengers which is a field collection with cardinality set to unlimited

I want to be able to, from hook_form_alter (or any other hook before the form is built), to set the number of "passengers" to appear in the "field_passengers" field of the form, to let's say 3.

Thank you for your help.