There's no set limit to the number of sensors you can pair with a single mobile device. Some users have a hundred or more sensors paired with a single phone. Similarly, a single sensor can simultaneously pair with multiple mobile devices.
If you plan to use your sensors in conjunction with a SensorPush WiFi Gateway, the same rules apply. There's no set limit to the number of sensors you can pair with a Gateway. Also, you can have multiple Gateways on one account. They can be in the same location to expand the wireless coverage area or they can be in entirely separate locations.