The thing about these limits is that the carriers and iPhone models are all different. For instance, the iPhone 5 should theoretically be able to handle up to 14 connections, but both AT&T and Verizon limits it to 5. As far as I have understood, it is hardware-limited, and therefore it is not affected by iOS version.
I have started on a list which anyone may suggest an edit to if they come up with new information. Feel free to comment as well.
- iPhone 3GS supports up to two peers combined through USB and Bluetooth-tethering. 1
- iPhone 4: 1, 2
- AT&T-version supports up to three Wi-Fi peers, three via Bluetooth, and one USB peer.
- Verizon-version supports up to five Wi-Fi peers, three via Bluetooth, and one USB peer.
- iPhone 4S supports up to five Wi-Fi peers. Unconfirmed
- iPhone 5 supports up to five Wi-Fi peers. (AT&T and Verizon confirmed 3)
- iPad 3rd gen is currently unknown.
- iPad mini is currently unknown.
- iPad 4th gen is currently unknown.
Please note that Apple have not disclosed any of these limitations, they simply state:
The maximum number of supported connections may vary depending on your carrier or iPhone model.
Apple have also stated in an article about all iPhone-versions and the third-gen iPad that:
Depending on your wireless carrier and, the maximum number of Wi-Fi connections may be limited to three at one time.