I would imagine about the same number needed for Lugia (11-12 people bare minimum). I've only done one Ho-oh raid so far, and there were 20 people in the lobby, so we were able to beat it. Surprisingly, even with 20 people, a Solarbeam Ho-oh actually roughed up a majority of my top Pokémon, so if I did it with 10 other people, it could have posed quite a challenge.
How many needed to beat Ho-oh?
Talking about with a group where most players are on the level 32 to 35 range.
How many have you all beaten it with?
Answers
The guys at PokeBattler seem fairly knowledgeable about these topics. And quote:
" Final note for those who think that the "casual group size" is still too low: hey, raids must be a challenge. If you just do a Google search or ask about what attackers/types are good against that raid boss, you can definitely win with those numbers. "
I did I did a Steel Wing/Solar Beam with 8 with about 50 seconds left on the clock. It was fairly brutal — I think Lugia with hydropump needs to be considered as a tank option for that moveset. I threw in a couple Golems to see if I could get a burst of high-DPS before they fainted, but they couldn't get a stone edge out before solar beam put them under.
For the second squad, I max-revived my two powered-up Raikous and my Zapdos and left the last three slots, including a Lugia, as the game's suggestions. If you want to get more than a couple charge moves out for each Mon, you're going to need a tank.
So, we just made 2 Ho-oh Raids in a row.
We all were between Lvl 37 and 40 and managed to beat them with 6 Accounts, with around half the time left.
The first one had Fire Storm, the second Brave Bird as charged attack.
Just Keep in mind, that Ho-oh flies very close, so you even have to throw a curved ball pretty straight to hit the center and therefor get a great or excellent.
Sadly both ran away, one of them with 2210 CP (maybe 96% IV) :S