Your pass band must extend to at least 50Hz.
Your stopband should be fully attenuating to your specification (you do have an unwanted signals specification, don't you?) by 1950Hz.
Where between those two limits you define to be the cutoff frequency of the filter (usually the -3dB point) depends on what order of filter you want to use, whether you want any excess bandwidth above 50Hz, and what your unwanted signal specification is. Some would advocate the geometric mean of Fpass and Fstop. It's a frequency that's between the two, so it has that going for it. It may not be the best to meet both your passband and stopband specifications, but it doesn't hurt to start there and see what you get.
I would choose a passband of a little more than 50Hz, and increase the filter order in a filter design program until the unwanted signals are suppressed to your specification (1%? 0.1%? 10%?). As the filter order changes, the ratio of useable passband to 'cutoff' frequency will change. As the passband is meaningful to your application, and the 'cutoff' frequency isn't, it's better to design with a passband, than with a cutoff frequency.