How many people for current Mewtwo raid assuming no relevant legendaries?

I wouldn't try it with less than 6. If everyone had lineups like yours, I'd feel comfortable with 8, as long as everyone max revived their best team.

I'd say 6 is the minimum. I had to try 3 rounds with 5 players because their line up sucks so hard, while they are 10 times as active as I am. I consider them to be the "average" player, therefore I would recommend a minimum of 6 for safety.

We did it with 6 today. I run a team consisting of Houndoom, Scizor, 2x Tyranitar, Hex Gengar, Shadow Ball Alakazam, all level 30-35. At least one person in the raid had a worse setup than me, while most others ran a lot of Tyranitars, probably having an overall better setup. I revived my team once, and we comfortably finished with 70 seconds left.

I would advise just building a team of 6 Ttars with Bite/Crunch and leveling them up to their breakpoints. It is the greatest non legendary(actually the greatest now that SB Mewtwo is no longer available) that you can use. Just make sure not to face focus blast. If you have 4 trainers with a full team of leveled up Ttars, and max revives ready, it’s possible. Just don’t face Focus Blast.

Gengar without Shadow Claw is not really worth the investment, and best used towards Ttars. I have taken out Mewtwo with 4 vs Psychic using teams of Ttar with about 33 seconds left.

I think 5 players with teams like this would suffice. 6 to be extra safe, but i'd be surprised if 5 didn't succeed with a few tries. If you had fog 4 too.

I think five is the right number, if the other teams are comparable to yours. I did it with five the other day, and that included two strong teams that knew what we were doing, one L35 that may or may not have been using an optimized lineup, and two lower level accounts that were not using ideal counters. It was close and took two tries, and it was against probably the easiest moveset (PC/P), but I think overall we had a weaker group than five of your team would be.

I was in a group that managed to 5-man the current mewtwo twice without any prior planning. No one else showed up but we decided to try it anyway and succeeded.

Both Mewtwos had Thunderbolt, was not weather boosted, 4 of the accounts had Ultra Friend damage bonus and all of us were level 30+ and had a decent number of Tyranitars. None of us max revived our first teams either. EDIT: This was in Clear weather.

We managed both with upwards of 20 seconds left on the clock.

A group of 4 isn't doable at your current strength probably. Everyone would need optimal counters or higher friendship. A group of 3 is only doable with like... 3 teams of 6 level 40 SC Gengar that need to be revived 3 times.

Definitely 4 is to low. With 5, Flamethrower and Psychic looks fine, PC/FB too( just spam Gengars). Ice Beam is risky, Thunderbolt and C/FB look hardest to beat. No any counters you have resist Electric and Tyras/Houndooms will be koed after 1 FB, While confusion eating gengars fast.

depend on the moveset. if the mewtwo use psychic, 5 are safe. 4 probably win. psychic are the weakest move against tyranitar.
because 4 days ago I experience playing with 6 players in total (last raid on exraid gym). which is 3 of them are my friend account (lv 29, 33 & 32) all only have several tyranitar boosted & non boosted from raid (none are power up) and the other 3 player are average level (lv 35, 38 & 39). and we finish the raid with 45 second left.
assuming that my friend account didnt have much damage (but all 3 are ultra friend). so its safe that 4 or 5 account with average level mid lv 30s are safe (at least ultra friend) against mewtwo with psychic- move only.

4 would be difficult with that setup, 5 can manage to win if you meet the right moveset for mewtwo
I am in similar situation, where there is a quite big community in my city, i live in rather quite part of pogo trainers. Have 10 member in our small group, but ussualy only manage 4-6 trainer, 7-8 on a good day to raid legendaries.
Try to do 2 mewtwo raid that show in 2 of our 3 gym in our area. Try with 5 trainers lv 26,27,29,38,40
The 38 and 40 pretty much are the big gun, using high level (34-40) counters like Gengar, Mewtwo, Tyranitar, Houndoom.
The 20ish they try to prepare counter as best as they can, but simply not strong enough yet. All three of them is a new trainer playing for about 2-3 month, but i still admire their spirit, and give some counter that i dont use or need anymore like tyranitar from raids, wild high level houndour ( evolve to houndoom change the moveset then trade), etc
Failed at the first raid which is C-FB, win at 2nd with PC-Psychic with 5 second left.
Hope that info help you..

5 should be easily doable if you improve your teams.

My girlfriend and I are away on vacation, and we "duo'd" a boosted mewtwo - what I mean is we had a group of 7 and none of the other 5 re entered after they fainted out, and we still won with plenty of time to spare.

That said, we're at best friends and both have teams a fair bit better than yours (e.g. I'm running a level 39 Shadow ball mewtwo, 39 Shadow claw gengar, 32 Shadow ball mewtwo, and three 32-34 bite crunch Tyranitar - reentering with two more level 30 Ttar, a level 39 SD/SE Ttar, a 31 dragonite, and a 39 and 35 kyogre.

We both got 3 damage balls (did > 20% damage individually) and her team is a bit weaker than mine. So I'd reckon I could do at least 25% on my own. I say 5 since you won't have a best friend bonus - you should be and to pull 20% weight each with good teams though.