Hi @Borja108 ,
You can set your listing to accommodate two guests and specify an additional charge for each extra guest per day.
In your listing description, you can mention that only guests under 13 (U13) can be added as extra guests since they will be sleeping on the sofa.
Potential guests can also review the listing photos to assess the sleeping arrangements and decide accordingly.
For more details, please refer to the following page:
Airbnb Help Center - Adding Extra Guests
Best regards,
Ahmed