You'll want to use the following syntax in your XLSForm. I suggest also added a hint and/or a constraint_message to tell the user why their answer may be rejected.
+------+------------+------------+----------+------------+-----------------------+--------------------------------------------------------+
| type | name | label | required | appearance | constraint | constraint_message |
+------+------------+------------+----------+------------+-----------------------+--------------------------------------------------------+
| text | incidencia | incidencia | yes | numbers | regex(.,'^[0-5]{5}$') | Must only contain numbers 0-5 and be exactly 5 digits. |
+------+------------+------------+----------+------------+-----------------------+--------------------------------------------------------+
For some additional related information see these posts:
Set Limit on Integer and Phone number and string length