Landon Winkler |
It would be +1 CR at most.
Keep in mind that two copies of the creature would be CR+2. And two creatures means double the hit points, double the attacks, and resistance against single target attacks.
In general, I'd say that it's a change within the CR. Unless the creature is already at the top edge of its CR, maximizing its hit points alone probably isn't enough to bump it over.
Cheers!
Landon