RC Phase Shift Oscillator
A RC phase shift oscillator is a type of electronic oscillator that uses a combination of resistors (R) and capacitors (C) to generate a continuous sinusoidal waveform. It is a feedback oscillator circuit that produces a phase shift of 180 degrees at the desired frequency.

Minimum Number of R-C Networks
In order to generate the required phase shift of 180 degrees, a minimum of three R-C networks need to be connected in cascade.

Explanation
Here's a detailed explanation of why a minimum of three R-C networks are required for a RC phase shift oscillator:

1. Phase Shift
The phase shift in each R-C network is determined by the RC time constant. In a RC circuit, the phase shift is given by the formula:
Phase shift = arctan(1/2πfRC)
where f is the frequency and RC is the time constant.

2. Required Phase Shift
For a RC phase shift oscillator to oscillate at a particular frequency, the total phase shift around the loop must be 360 degrees or 2π radians. Since each R-C network contributes a phase shift of approximately 60 degrees (π/3 radians), a minimum of six R-C networks would be required to achieve a total phase shift of 360 degrees.

3. Feedback Loop
However, in a practical RC phase shift oscillator, the feedback loop is broken at one of the R-C networks. This is done to prevent the circuit from becoming unstable and to ensure that the output waveform is sinusoidal. Breaking the loop reduces the total phase shift by 180 degrees (π radians).

4. Total Phase Shift
With the feedback loop broken, the total phase shift required is reduced to 180 degrees. Since each R-C network contributes a phase shift of approximately 60 degrees (π/3 radians), a minimum of three R-C networks is sufficient to achieve the required total phase shift of 180 degrees.

Conclusion
In summary, a RC phase shift oscillator requires a minimum of three R-C networks connected in cascade to generate the required phase shift of 180 degrees. The combination of resistors and capacitors in these networks determines the frequency of oscillation, while the feedback loop ensures stable operation and a sinusoidal output waveform.