I'm a little unsure as to how to apply the monks Flurry of Blows ability to a creature with multiple limbs.
To simplify how many attacks would a human who had 4 arms get? How would one calculate the attack bonuses to include extra limbs?
Flurry of blows provides a single extra attack, as per Two Weapon Fighting; the number of attacks a monk gets while using flurry of blows is not linked to a monk's number of limbs, but instead to his base attack bonus. In short, extra limbs would have no effect.
Yeah, they have it where when using Flurry of Blows, it's assumed you're using ALL your body parts to land whatever hit you can, knees, elbows, headbutts, fists, tail, etc.
I still use 3.5e's method, however. In 3.5e, you used your Flurry of Blows as a "primary" attack. You can still use an "off-hand" attack and get another Unarmed Strike in and take TWF penalties (and, of course, build off of it with ITWF and GTWF). Then, if you had natural attacks, you were allowed to use those as "secondary attacks" and take the usual penalties for attacking with secondary natural weapons. Using FoB used up ONE of your "hand" attacks, so if you had two claws or four claws, or only 4 tentacles, or whatever, one is used for the FoB.
It's all spelled out in Sage Advice FAQ on their website. Made stuff like Marilith Monks or Mind Flayer monks pretty lethal.
Which meant, yes, using a Marilith Monk 1 as an example, she'd use up one of her 6 arms to do a FoB at -2 penalty to all attacks. Having Multiweapon Fighting for free, she'd be able to use all her other arms for an extra attack, at no additional penalty (and get even further with Improved and Greater Multiweapon Attack feats). In addition, she can then take -5 penalty (or -2 with Multiattack feat) to throw in her tail attack.
Marilith Monk with all those feats, on a Full Attack, can get off 21 attacks.
I had a friend with a LN Druid/Monk that planned on wildshaping into animals and using FoB for Unarmed Strikes, followed by secondary attacks with bite or claws in animal form.