You can make one flurry with two strikes
Flurry of Blows is not limited by a number per turn, but by coupling it to taking the Attack action on your turn, and by your available bonus actions:
Immediately after you take the Attack action on your turn, you can spend 1 ki point to make two unarmed strikes as a bonus action.
So, if for example you have a multiclass fighter/monk who has Action Surge, they can use it to take a second Attack action in the same turn. However, since Flurry of Blows is a bonus action, you still cannot make multiple Flurry of Blows, because you normally only have one bonus action per turn, and Action Surge only gives you an additional action, not also an additional bonus action. p. 189 PHB:
You can take only one bonus action on your turn, so you must choose which bonus action to use when you have more than one available
It also only allows you to spend one ki to make two unarmed strikes as a bonus action. There is nothing in there that says you can spend 2 ki to make four attacks as a bonus action, or 3 ki to make six attacks. It would need to say something like "For every ki point you spend, you can make to additional attacks; all these attacks are made as your bonus action". It does not say anything like that.