It gets extra attack and extra reactions per head.
Are you treating the Hydra as naturally starting with more than 5 heads, or are you assuming it already was fighting?
Already fighting: Are you reducing the HP by 5 for every extra head? (head damage typically costs it 25 hp to remove a head, then +10 hp for each head regrown, so to gain one extra head, it should be down 5 hp.)
Naturally more heads: Are you increasing the HP by every extra head it starts with? +25 HP is a significant boost along with the extra attacks and extra reactions.
Case 1) already fighting, starting with 6+ heads AND you are reducing the HP by 5 hp per extra head. No change in the CR. It is in certain ways, weaker, so no CR change.
Case 2) Naturally more heads and you increase HP by 25 per extra head. I would rule that +1 CR per extra head.
Base Hydras start with 5 heads. Simple question. How much would adding heads at the start alter the CR of the monster by each head?
It gets extra attack and extra reactions per head.
Are you treating the Hydra as naturally starting with more than 5 heads, or are you assuming it already was fighting?
Already fighting: Are you reducing the HP by 5 for every extra head? (head damage typically costs it 25 hp to remove a head, then +10 hp for each head regrown, so to gain one extra head, it should be down 5 hp.)
Naturally more heads: Are you increasing the HP by every extra head it starts with? +25 HP is a significant boost along with the extra attacks and extra reactions.
Case 1) already fighting, starting with 6+ heads AND you are reducing the HP by 5 hp per extra head. No change in the CR. It is in certain ways, weaker, so no CR change.
Case 2) Naturally more heads and you increase HP by 25 per extra head. I would rule that +1 CR per extra head.