Is there any limit to the number of users connected via Personal Hotspot?
Quoting from an Apple support article about Personal Hotspot (emphasis mine):


  When you connect a device to your Personal Hotspot, the status bar turns blue and shows how many devices have joined. The number of devices that can join your Personal Hotspot at one time depends on your carrier and iPhone model.


I believe the typical limit is usually between 3 to 5 devices.