Ok, so after some research I do find a way to do what I need.
I replaced the spinner field by selection items and now, with some javascript function, I'm able to show and hide as text field as I want.
This is my Javascript function if this can help anyone later.
addFront : function(){
var valfront = new String;
valfront = ca_fdGetSelectedOptionValues(ca_fd.formId,'idnbfront');
for (nFront=1; nFront<5; nFront++) {
if ( nFront <= valfront )
{ca_fdShowField(ca_fd.formId, 'idfront'+nFront);}
else
{ca_fdHideField(ca_fd.formId, 'idfront'+nFront);}
}
},
Thibaut