Most attacks vs 1 target in 1 turn: 22? Can anyone do better?

[MaxWilson]

I agree that this is almost certainly the way to the most attacks, but I will raise a couple of objections.
I would like to point out that the Hydra doesn't have a method of healing itself (No allies is assumed), and the net damage per Hydra head is 15 (Take 25, heal 10). A Hydra has 172 hp, so you can only take the damage 10 times, for a net total of 15 heads, which becomes 30 attacks with action surge, and 31 with a bonus action attack from some means. (Battlemaster maneuvers come to mind). With Second wind, which you have from fighter levels you should be able to get one more head, and 2 more attacks.

In the case of Shapechange and True Polymorph, you're going to have to make concentration checks for each one, and for True Polymorph you don't have action surges. True Polymorph and Magic Jar as permanent buffs that you had prior to this feels against the spirit of it.

As a last point, you don't really have a reliable way to deal exactly 25 damage to yourself. This certainly seemed to be a point of contention for the OP in the samurai case, and if you do figure a way to deal exactly 25 damage to yourself each time, we're still talking about at minimum 12 rounds of setup, when the other greatest amount of setup is 2 rounds.

You don't need action surges when you have arbitrarily many heads, nor do you need to deal exactly 25 damage to yourself. Just bite off one of your heads, grow two more (and regain 20 HP), rest when you need to regain HP (can precast Goodberry as a Bard 17 if you want to jumpstart things by growing a few dozen heads right off the bat). Rinse and repeat until you have thousands of heads, if you want.

As far as setup time goes, how many heads could a character grow in the time it takes someone else to go from level 17 to level 20? You require weeks or years of setup time to hit 20th level and 23-35(?) attacks, but as a hydra Eldariel can get 100 attacks in only a few hours of setup.

Of course in practice you'd never do this because it's pointless overkill, fencepost security--like putting up a wall a thousand feet high and green feet long. 3000 heads is no better than 100 heads for overcoming Dispel Magic. You'd be far better off True Polymorphing into an Atropal instead and summoning 3000 Wraiths instead.