The USB-ProxSonar-EZ2 has a detection zone of ~3 feet and will allow for 10+ simultaneous sensors.

These ultrasonic proximity sensors detect objects in a calibrated detection zone. The USB‑ProxSonar‑EZ sensors report when an object (such as a person) enters the detection zone. The target is released after leaving the USB‑ProxSonar‑EZ detection zone.

Component Overview
  - USB interface
  - ~2.5 second target acquire time*
  - ~1.5 second target release time*
  - Automatic multi-sensor operation
  - People sensor
  - Resolution of 1 inch
  - Maximum Range of 125 inches (318 cm)
  - Detection zone range of 36 inches (91 cm)
  - 42kHz Ultrasonic sensor measures distance to objects
  - RoHS compliant
  - Virtually no sensor dead zone, objects closer than 6 inches range as 6 inches
  - Small, light weight module
  - Designed for easy integration into your project or product
  - Good balance between long and short range detection zone
  - Volume Pricing (25 or more units): Contact MaxBotix Inc.
  - *Custom times available for a nominal NRE fee