Do I need to use two AirPort Express to extend my wifi-network range or I only need one AirPort Express to directly extend the network?

The airport express is cheaper alternative to extend your network but will work just as good.

If the wireless router you want to extend is another AirPort Extreme, Express, or Time Capsule, you will only need ONE AirPort Express base station to extend it.

If it is not, then you have two choices:

1. If your goal is to have a wireless connection between base stations, then you will need a minimum of TWO AirPort Express base stations.
2. If your goal is to have a wired connection between base stations, then you will only need ONE AirPort Express base station.