[RPG] Is Flurry of Blows limited to one target

Yes, as per Player's Handbook FAQ

Found reference here:

Attack powers that have a target line and deal damage are attacks (See: Magic Missile). Ergo, Flurry of Blows is an attack.

This is clarified in the FAQ for Player's Handbook, in case anyone was wondering where it's coming from.

And the relevant original text is here (item 38):

1. What happens if I use magic missile while benefiting from a power like greater invisibility? Does it count as an attack and would my invisibility end?

Yes, it does, and yes, it would. The initial use of any attack power that has a target line, an attack line, or both counts as making an attack. Because of this fact, using an attack power like the fighter's rain of steel does not count as making an attack, since the power has neither a target line nor an attack line

No. The Attack action is a specific action that you take in combat, which can include multiple attacks. From page 71 of the Player's Basic Rules:

Attack

The most common action to take in combat is the Attack action, whether you are swinging a sword, firing an arrow from a bow, or brawling with your fists. With this action, you make one melee or ranged attack. See the “Making an Attack” section for the rules that govern attacks. Certain features, such as the Extra Attack feature of the fighter, allow you to make more than one attack with this action.

The important distinction to understand here is the difference between "the Attack action" and "an attack". Flurry of Blows can be used after you take the Attack action, not after an attack.

So you take the Attack action, using Extra Attack to attack twice, then you can spend a ki point and use your bonus action to attack twice with Flurry of Blows.

It should be noted that even if Flurry of Blows could be used after an attack rather than after the Attack action, it still requires a bonus action to use, and you can only use one bonus action per turn.