how many bones per mesh in unity iphone?

1. No limit (until you run out of memory, at which point your app will be terminated. But that isn't likely to happen because of bones.)

2. No limit (although 4 x the number of vertices is the highest number that can affect the mesh.)

3. No. Not in regular Unity, either.

Bones can kill the performance of your app if you use too many, but although the performance can be terrible, the game will still run. It will just be a slideshow.

The docs suggest <30 bones per mesh and mesh sizes of 300-1500 polys. Of course if you want loads in view at the same time the fewer the better.

There's more detail in this Unity doc link:

http://unity3d.com/support/documentation/Manual/Modeling%20Optimized%20Characters.html

We are using 18 bones per mesh in our project and can have around 10 mobs visible without the frame-rate plummeting. This is on a 3G S and mobs were at variable distances. I've banged about 25 of the rigged worker guy with the wrench (comes with Unity) onto the same phone and had around 20fps, which is too low really. Our 10 mobs was more like 30fps which is fine for our slower paced RPG.

Hope that helps!