What is the required LPF order for a Signal Chain?

In a signal chain the order of a lowpass filter, LPF, depends on the ADC sampling frequency, fs, the input signal bandwidth, SignalBW and the magnitude of out-of-band signals or noise producing aliasing errors. The highest aliasing errors are out-of-band signals or noise in a frequency zone around fs and equal to 2xSignalBW (a worst-case aliasing zone is fs ± SignalBW). Any signal in the aliasing zone will fold back into a lower frequency and mix with the input signal.  

A LPF before the ADC will attenuate signals or noise in the worst-case aliasing zone. A LPF order is the number of 1st and 2nd order LPF stages. For example, one RC filter is a first order LPF and one active RC filter is a second order LPF. The higher the LPF order the higher the stopband attenuation in the aliasing zone. In an oversampled signal chain, fs is much higher than SignalBW (fs at least 10xSignalBW) a 2nd or 3rd order LPF provides adequate attenuation at ≥fs (-40dB and -60dB for a 2nd and 3rd order Butterworth LPF respectively). In case fs is very close to SignalBW, for example fs=5xSignalBW, then a 4th or 5th order LPF maybe required to provide at least -50dB to -70dB attenuation at fs. 

The most useful guide to the analog filter order for a signal chain, is the magnitude of out-of-band signals. In case where the input spectrum is limited to a very low bandwidth, a 1st order RC LPF is adequate.  

Go to the ADI Filter wizard tool to design your Low Pass Filter. https://tools.analog.com/en/filterwizard/