how many devices can be tethered to the iphone5

I haven't tested it, but according to the Subnet Mask (255.255.255.240), you should be able to do 14 Wi-Fi devices stimutaneously. This is on my iPhone 5 with ATT

I would say the practical limit will be set by the bandwidth your carrier is giving you. After all, it works like a pie. Each tethered device gets a slice, the more the thinner.

The maximum number of supported connections may vary depending on your carrier or iPhone model. As an example, both Verizon & AT&T limit this to 5 devices.

That is the limit for the iPhone 4 which I used to have

As of today, AT&T informed me that the iPhone 6 and iPhone 7 will allow up to 10 devices tethered simultaneously.