Number questions are directly mapped onto <xforms:input> elements. Default values are directly written into the XForms instance data. The length attribute is encoded in the XSchema declaraction.
QUESTION "Number question"
LEN (12 3)
DFT 42
ANSWER number_variable
<xsd:schema targetNamespace="http://example.info/types">
<xsd:simpleType name="question0">
<xsd:restriction base="decimal">
<xsd:totalDigits value="12" />
<xsd:fractionDigits value="3" />
</xsd:restriction>
</xsd:simpleType>
</xsd:schema>
<xforms:instance>
<response xmlns="">
<question0>42.000</question0>
</response>
</xforms:instance>
<xforms:bind nodeset="question0" id="question0" type="type:question0" />
<itp:question>
<xforms:input ref="question0">
<xforms:label>Number question</xforms:label>
</xforms:input>
</itp:question>