There's currently FUD around what's a safe batch number to use with cumin when issuing puppet runs on many hosts. Achieving a fast puppet run is especially important in emergency situations, for example when distributing new policies/ACLs/etc. Said number must strike a balance between being high enough (i.e. fast propagation of puppet changes) and not make the puppet run fail (e.g. overloaded puppet servers).
For context, in the past we've run into problems where too many concurrent puppet runs would overload the puppet servers; we've limited the mod_passengers concurrent workers to mitigate this problem. The exact effects of many concurrent puppet runs needs to be determined though (e.g. agents queue waiting for their turn? apache / passengers makes the puppet run fail, etc)