Found in the CRD about the dead pile
Dead pile
If you have more than one copy of a unique companion in your dead pile, you can't play one of them from your dead pile.
Found in the Comprehensive Rules 4.0
The Rule of 9
You cannot have more than nine total companions in play and in your dead pile at any time. (Each copy of a companion in play or in your dead pile counts as a separate companion, whether it's unique or non-unique.)