For example would it make a difference if I change Audio Decimation to 1 in WBFM Receive and change the Decimation to 50 in Rational Resampler block?
Why don't you try? GNU Radio lends itself very well to experimentation, and by attaching signal visualizations (mainly, frequency sinks), you could combine your theoretical with your practical knowledge.
However, this will not really work. For frequency modulation, you can't have a input signal bandwidth equal to the output signal bandwidth (decimation is the ratio $\frac{\text{input sampling rate}}{\text{output sampling rate}}$); in "which" spectrum would the modulation take place?
I'm also having a lot of doubt regarding your flow graph: It won't work at all.
If I trace back your sampling rates from the Osmocom source, then then you have
- 20 MHz (after osmocom source) ->
- 20 MHz / 100 = 200 kHz (after decimating Low pass filter) ->
- 200 kHz * 12 = 1.4 MHz (after rational resampler) ->
- 1.4 MHz / 10 = 140 kHz (after WBFM receiver),
but your Audio Sink expects 48 kHz. That can't function!
Your question shows that you might want to brush up your knowledge on what FM is, and how signals are represented in digital signal processing.
I've seen you ask similar question; I'd strongly suggest going through the official guided GNU Radio tutorials. They really explain a lot of the concepts you're missing here. Also, things like "Great Scott's Gadgets introduction to complex numbers (signals)" is a great starting point for SDR adventures!