Multiple Wireless Sensors to an Arduino
I would recommend the nRF24L01.

You don't need one pipe per node - just one pipe. 

Each pipe is effectively an address that a node responds to - having multiple pipes means you can have multiple addresses on one node (like aliases) - but of course you only need one address per node, and hence one pipe per node.

The range is more than adequate (with the antenna version I have managed to get hundreds of meters no problem) and the price is so low as to be negligible.