Detect position of walkers along a path using IR transceivers
Use resistors spaced apart by a factor of at-least 2 

eg: 1K, 2.2k, 4.7K, 10K, 22K, 47K, 100K, then combinations will non-ambiguous.

7 resistors per 10-bit ADC input is about all that seems likely to work well. use 680 ohms for the fixed resistor.