Multihands does not benefit unarmed strikes
In terms of pure mechanics, using multiple hand on a single attack is no different to a single unarmed attack. You gain no damage bonus and it does not count as a 'multi-attack'. From the basic rules for melee attacks we have the following exert:
Instead of using a weapon to make a melee weapon attack; you can use an unarmed strike: a punch, kick, head-butt, or similar forceful blow (none of which count as weapons). On a hit, an unarmed strike deals bludgeoning damage equal to 1 + your Strength modifier. You are proficient with your unarmed strikes.
And from the Monk's Martial Arts feature:
- You can use Dexterity instead of Strength for the attack and damage rolls of your unarmed strikes and monk weapons
- You can roll a d4 in place of the normal damage of your unarmed strike or monk weapon. This die changes as you gain monk levels
These as the rules that you care about when making an unarmed strike. You can describe the attack however you like. "A punch, kick, head-butt, or similar forceful blow" such as a triple-punch are all considered equivalent. As a monk you can either use Strength or Dexterity for these attacks and you deal 1d4 + modifier damage. This dice increase at 5th, 11th and 17th level.
Thematically though you can describe your attack as multiple fists striking at once. You can do this for each of your unarmed strike, even the additional ones from Flurry of Blows. I can see some very cool descriptive combat from a multi-armed monk.
Where it does benefit you
The mechanical benefit of your Multihands feature could be useful for a Way of the Kensei Monk. As one of the few monastic traditions that rely on weapons for their attack you would be able to wield both a melee and ranged weapon simultaneously.
Essentially the main benefit of this feature is to allow you to be wielding more weapons at the same time, giving you more options on your turn. You could carry a heavy weapon and use Great Weapon Master one turn. Then switch to a Longbow on the next without using an item interactions. All this while getting the AC bonus from a shield (not when playing as a Monk like you are though).
Getting Multiattack as a PC
Gaining multiattack as a PC is extremely difficult. No class gets it as a feature and there are no feats you can take. Note multiattack is distinctly different from Extra Attack, the class feature which Monk (and others) get. One way you can obtain it is by becoming a lycanthrope, as they get multiattack.
If you do manage to get it, remember that Multiattack is not the same action as Attack and does not qualify as a trigger for your Flurry of Blows feature which requires the Attack action.