One.
One hit point is the right number of HP to make a minion work right.
There's been a lot of complaints about minions having 1 hit point over the years. I find this interesting, since there was a time when NPC's and low-level monsters could easily have 1 hit point (or such a meager amount that it was irrelevant how many they had).
I mean, I think it was the 2e DMG that said a town blacksmith might have 1d8 hit points, and I routinely had 1st level Fighters with exceptional Strength and weapon specialization at tables who couldn't fail to kill an 8 hp character with a single attack.
I mean, if you want minions that have a chance to not die in one hit, you can give them a mechanic like that- zombie minions, as I recall, had a chance to not die. They couldn't take half or miss damage from attacks. You had to hit them. And while I heard complaints about "oh a Wizard stabbing them with a dagger could kill a 20th level minion", no Wizard was going to be doing that.
They would have scaled-up At-Will attacks backed by a high Intelligence and a magical implement, lol. But somehow, in a span of 20 years, we went from people thinking it was wonderful that a 1st level Wizard could be threatened by a house cat to not being able to wrap their heads around a cinematic concept like easily-dispatched mooks (seen in all forms of media everywhere).
But that was 4e's problem in a nutshell. It presented itself as a game, and took few steps to wrap it's mechanics in even a facade of verisimilitude. And no matter how unrealistic D&D is at it's core, a lot of people prefer the illusion.
A couple years back, there was an argument about minions that had the ability to automatically inflict damage due to an aura. There was nothing wrong with the ability, it was something other creatures did- the complaint was simply that there was no explanation for how it functioned! Were the zombies possessed by angry spirits of rage and hunger that lashed out at victims? Who knows, it doesn't say!
But if you said "a miasma of corrosive energies surround these zombies", it would have been all good.
I think most of the things that people took offense to with 4e came down to not having a coat of paint on them. Like if an ability said "your blows are so forceful, they can batter down the strongest defense", maybe nobody would have batted an eye at half damage on a miss for Martial attacks?
Back to minions, a DM I knew had elaborate house rules for minions that gave them extra hit points, as he felt that made more sense. When I pointed out to him that they still died in one hit every time, he got angry with me, saying "that's not the point!". Somehow, it wasn't the fragility of minions that bothered him, it was the exact mechanic that allowed them to be able to be easily dispatched.