7 on 40 is... not actually bad odds. That's slightly more than 5 on 1, and the Kenobis aren't good enough to defend against force lightning or TK very well, nor is he particularly tactically wise yet.
Put a Krayt surrounded by 7 Kenobis- far weaker versions of a master he personally knows and has fought- and a few seconds later you're going to have 7 dead Kenobi's and he's going to move on.
Heck, are the Kenobis going to be able to even resist the illusions? Wyyrlok and Maladi may have them killing each other.
Yes, strength of numbers matters. But this isn't 1 on 20 or some other such one-sided thing.
It's 7 combat masters, including the Dark Lord of the Sith and his only-moderately-weaker second in combat, vs 40 padawans who are going to have trouble dealing with their force attacks.
If Wyyrlok manages to illusion, say, 5 Kenobis, that effectively takes out 10+ of them.
5:1 isn't that bad when it's padawans vs masters, especially not when the masters can guard each other's backs, multiple like to carry paired sabers for fighting multiple foes at a time, and multiple specialize in force abilities the likes Kenobi has yet to encounter.
Replace the 7 One Sith with, say, Dooku/TCW Maul/Savage/Asajj/TPM Maul/Grievous/Aurra Sing and they'd manage too, even having less sorcery and force attacks.
What makes up the number and the precise numbers differential both matters.
If this was Clone Wars Movie Kenobi, the One Sith would be dead. If this was RotS Kenobi, they'd be incredibly dead with few casualties in return.
But this is padawans.
Even a talented padawan nearing knighthood isn't *that* good (unless their name is Skywalker), and 5 vs 1 is a number people have fought against and won. Antares Draco's beaten 7 to 1 against sith warriors, he just looked like crap by the end.
Krayt, as we've seen, can beat 4 people when surrounded in a second, and that was pre-reborn sick Krayt. He can do that a couple times here. And even if he or someone else does get hurt, he can heal anyway so endurance isn't as much of a factor as it can be with numbers sometimes.
Wyyrlok, he has his illusions and this Kenobi is still a hot head who'd fall for them. He gets 4-5 Kenobis to think other Kenobis are really Sith, that keeps 10 or so busy pretty easily.
5:1 vs Padawan Kenobi is not horrible odds when they can't even fully surround individuals.
TPM Kenobi may not be great in the grand scheme of things, but he's still a council-level duelist and a reasonably powerful force user. I doubt any of these guys would be victorious in a 5 on 1 confrontation except perhaps Darth Krayt. (emphasis on "perhaps")
Since when is *Phantom* Kenobi a council level duelist? His presence didn't even have much effect on how Maul was doing against Qui-Gon, and he was kicked in the head three times during the fight. Aurra Sing also didn't have much trouble kicking him when she wanted.
TPM Maul could take multiple TPM Kenobis fairly solidly, I feel.
Episode 2 Kenobi was a council level duelist. Phantom Kenobi was noticeably lower than that, he grew a lot in the interrum.
Something else I wanna emphasize on Phantom Kenobi- he wasn't a Soresu master yet. Indeed, he's known for being reckless and aggressive, and the numbers are likely to make it even more-so, which is going to increase the openings he gives and the casualties the Kenobi's take due to their overconfidence!