By default Magento only has one filed to input the recipient of the contact form (in admin).
If you would like to get this sent to several email addresses then a small hack is required as Magento adds some encding to address in this field.

Use

mail@example1.com>, name <mail@example2.com

to send to two recipients. name is the name shown as the address in the second mail.

To send to 3 addresses use

mail@example1.com>, name <mail@example2.com>,name2<mail@example3.com

To sent to 4 or more, just add more as in the 3 addresses example