You should have 39 heads.

I think my DM is going to throw one of these as the first boss in the campaign I'll start next sunday. (Chars start at 5th level)

I'll come back to tell ya'll if I'm right.

You're supposed to roll over damage, as it's still treated as the same creature. So, it's possible to kill one quickly enough before the heads regenerate. All depends on party's DPS/level and rolls. 

You're not counting the fact that one head has to go each time you grow another. So if it starts with 5. You chop of one, making 4. then add 2 more, 6. And it deals 5 damage because it regained some. So, 39.

I am a hydra  in my game my friend turned me into it with polymorph

Best Game Ever

Odd that the art has seven heads but the description says five.  That said, nothing wrong with seven!

Perhaps if you roll a good enough number and you have a kind DM you could trick the hydra by tossing food near one head so that the other heads attack the one piece of food.

Then I'd say they must be behind the hydra and treat it as having half cover from the flailing heads. If they get a 15 or 16 they still hit, but they hit a head instead.

just a tip  i think it is to powerful for 3rd level 

Quote from 283leis >>

Assuming each turn the hydra takes exactly 25 damage, and 0 fire damage is done to it throughout the fight, what is the maximum number of heads a hydra could get before it dies?

 if you heal the hydra it could get an infinit number of head(probily become incubern eventuly though)

If a Hydra takes 25 damage from 5 different characters in one round before it takes it’s turn- it would die, right? Meaning it would essentially have only 125HP in that round. Of course, the party would need to be really good.

 What do you think would happen to 3 seventh level characters versus a Hydra? (Rogue, Fighter, Healer with necklace of fireballs) 

The hydra regains 10 hit points for each head regrown in this way.

Since the Hydra regrows 2 heads for each 1 lost, does that mean that it heals for 20 hit points?

Its challenge rating should be higher

Quote from Agathon >>

is it me, or does this seems rather....underwhelming?

 Agreed, I feel like it should have legendary actions

It should have something. Hopefully a DM puts this thing in 10-15ft of water to help it out.

This sounds like the perfect situation for FIREBALL!

”Use Fireball and only Fireball. Nothing but Fireball. Just Fireball. Just Fireball. JUST FIREBALL.”

-JoCrap

Quote from Zoogleplex >>

The hydra regains 10 hit points for each head regrown in this way.

 

Since the Hydra regrows 2 heads for each 1 lost, does that mean that it heals for 20 hit points?

 Yes I believe so.

I've always wanted to use a hydra, I even have two minis for it, but my issue is that since fire damage is so common, and even a torch as an improvised weapon could do at least 1 point of fire damage, it seems kinda wasteful. Like almost every party has at least 1 fire based dealer. I know in my current group have have at least two for sure. So I'm thinking I'm going to take the fire damage part out, and replace it with another kind... keep the party guessing and on their toes, but also have it be something they could use. I'm thinking acid, but I'm not sure? Any thoughts?

My friend put this in a campaign and was found by 5 first level characters. THEY WON!