Minimum for a Ho oh raid

5 is reasonable for non-SB, assuming players above level 35 with Golems. For SB, I'd say 8 or so.

Non SoB should be about 5, as 'gibell' said above...as long as everyone has a solid team.
We did SoB with 7, but we all used 2 teams - you can get the damage done, but so will Ho-oh. My first team was Nites Raikou and Moltres, and went in with a team of Golems to finish the job when my first team fainted

5 isn't hard and there is a report of a 3 man (easiest moveset and big golems)
7 if it has solar beam seems to be a safe number

https://www.reddit.com/r/TheSilphRoad/comments/7cobnz/a_more_realistic_suggested_group_size_for_raids/

The spreadsheet here from the guys at PokeBattler is a good starting point to estimate if you have enough players for a raid. They predicted for Ho-Oh a minimum 3, realistic 6, and casual 9.

If you have trainers with the appropriate mons it isn't hard. Yesterday we struggled with 8 SW and FB which shouldn't be possible. My friend and I are 38 and the other 6 were low 30s and we noticed they were using Arcanine and Lugias...

No luck with the solarbeam one
But manage to do it with 4 trainer against the fireblast Ho-Oh
All trainer in level 40, with good level attackers to back it up (lv 33 to max level mons)
No full high level golem though, we still use some tyranitar SE, Gyara HP, and Vaporeon altough they are mostly a former old gym defender so atleast they all almost at its max CP
That's make us all had to rejoin and win with only 10 seconds left

Yesterday 5 of us did a SW/FB ho-oh raid with 35 secs left. All players 36-40 with good counters. I have yet to face a SB ho-oh, so cannot tell from my experience. My guess is 6-7 solid players for SB , 5 for non-SB