When the base power says you can't activate this multiple times does that mean you can only heal 1 strain per check?

Correct, unless you have purchased the strength upgrade. That can be activated multiple times

18 hours ago, TheShard said:

When the base power says you can't activate this multiple times does that mean you can only heal 1 strain per check?

Yep. As Varlie noted, you need the Strength Upgrade to heal (or inflict) more than 1 strain per check, and that upgrade can be activated more than once.