How many players are needed to beat Heatran?

Using Pokebattler, Groudon has a TTW of 502.4s and Rhyperior has a TTW of 538.7s against a Heatran with an unknown moveset. These are at level 30. Those numbers are below 300 when cut in half, indicating a duo is possible using Groudon and Rhyperior. Even Rhydon is acceptable for a 2-man as long as it's not against Iron Head sets.

Game says 20 players to win. I am guessing that you can do it with less.

Just did a raid successfully with two level 40s and three level 30 to 33. It was close, but it was Iron Head and no one had any of the supreme counters. So a more well-equipped group probably could do it with a couple fewer.

Better to use level 30 groudon or level 40 kyogre or level 40 machamp?

Two (against Stone Edge or Fire Blast). GoBattleSim gives 100% odds in that fight in extreme weather (96% if using Rhyperior against Fire Spin/Fire Blast). Three against Iron Head.

Used perfect level 30s in simulations.

Completed a raid with 6 players just now. All high levels with good counters (Groudon, Ryperior, Kyogre). Went down fast. I guess 3 players with best friend status can do it easily.

I did level 40, 38, 35 best friends at 7pm. Against Iron Head and partly cloudy. Finished with 97 seconds left. Used ground/Groudon but also some water (more Kyogre/Gyarados on the 38 and 35). Big account has level 38 Groudon, 3 level 30 Groudon, two max Kyogre—that party fainted no dodge after 200 sec of battling, finished 2-3 sec after that. Caught all 3 FWIW, 16-18 balls makes that somewhat simple.

It looks pretty doable with two strong, optimized accounts. Not even perfectly optimal, just really good teams with a few Groudons/Rhyperiors, and filled out with Rhydons and other decent counters. That's assuming Best Friend bonus.

Three should be pretty easy.

It should only take two I just beat a weather boosted heatran with my wife about an hour ago

If my son's account can be prepared for a trio with 6 level 32 Machamp (2 of which have CC), I wouldn't worry if you, a friend, and a brand new account all show up.