Create a new number field, please follow the same steps as above and take into account new options which are explained below:
You can see Min and Max fields. This is a possible range for this field. The plugin will not allow you to add more than 10 if you use the configuration like on screenshot. Also you will see Step field - it is a number of units the field value can increase/decrease by.
The Search range option will allow you to add min - max fields in Search widget like for the Bedrooms/Bathrooms fields.