From the Comprehensive Rules 4.0:
A sanctuary is any site 3 or site 6 on the adventure path. Sites preceding the Shadows expansion set mark this with a keyword.
At the start of your turn when your fellowship is at a sanctuary, you may heal up to 5 wounds from your companions (not allies). (See healing.)
You may choose to heal 5 different companions once, or one companion twice and another three times, or any other combination. You don’t have to heal any wounds at all – you may choose any number from zero to 5.