XMega External Real Time Oscillator Capacitor Values
Now what do i do, if i want to use a oscillator with a load
  capacitance let's say 12,5pF? Would i use two external 4,5pF Caps in addition


Roughly speaking, yes. But the datasheet example has certain assumptions which might not be true for your own board. They assume that 8 pF is already somewhere on the board.

First, you need to estimate the actual capacitive load from (a) traces and pads at each end of crystal, and (b) add an input capacitance of the IC itself. 

The parasitic capacitance can be either calculated using PCB development tools for given pad and trace width/length and PCB stackup (distance from signal layer to underlying ground, if any), or can be simply measured on empty non-populated PCB.

The parasitic pin capacitance should be taken from manufacturer's datasheet, or it also can be measured, but it is a quite challenging task.

Once you determine your actual capacitance of traces/pins, you can select the external cap's value by following logic:

If your crystal requires 12.5 pF load, it means that each end should be loaded with 25 pF caps (since the caps topology is "in series", so 25+25 = 12.5). Now subtract your determined parasitic capacitance from 25 pF, these will be the caps you will need to solder.

If a really high tolerance of oscillator is needed, the final caps value should be determined experimentally under fully-powered board and using extremely-low-capacitance probes.