I've heard about the 5 computer limit. I'm wondering if iOS devices are included in that or not.
There is no limit to how many iOS devices you can sync with iTunes.
The 5 computer limit is how many computers you can have authorized simultaneously with iTunes.
I'm pretty sure this has changed now. Apple updated their policy in August '11.
I've had no issues syncing 300 devices with one library.
Other than disk space to track the backups of each device, your time and money, I don't think there is any pre-defined limit.
No, iTunes has no limitations for iOS devices. You can use as many devices to sync you want.