The LM336 type you have is a shunt voltage regulator capable of regulating between 4 volts and 6 volts.
What is the lowest and highest possible voltage at the input
Because the LM336 is a shunt regulator it means that the input voltage is also the output voltage i.e. between 4 volts and 6 volts.
I see an example of the chip being connected to a 10V source via a 5k
Providing you use an input resistor that adequately limits the current to 15 mA then you will be ok. Check that at higher temperatures the 15 mA figure isn't degraded of course.
For instance you could run with an incoming voltage of 100 volts but the resistor will need to drop 95 volts and, if you want an operating current of 15 mA then the resistor value will be 6333 ohms and dissipate nearly 1.5 watts.
But you don't need to operate at 15 mA - the LM336 will work at 0.6 mA and this means a resistor value of 158 kohm and a power loss of only 57 mW.