If I want to know average value of deposited energy in full volume of target, should I set number of bins equal to 1 ?
If your target is a RPP or a cylinder, you can overlap the respective USRBIN grid type consisting of a single bin and score ENERGY. This way you will get a single number representing the average deposited energy density over the target [GeV/cm3].
Otherwise, assuming that your target is a geometry region, you can use USRBIN by region and get the total energy deposited over the target [GeV], to be further divided on your own by the region volume [cm3] in order to obtain the above.
The total energy deposited over each region is also given, for any cycle, in a dedicated region table of the output file (.out).