If the sampling rate meets the nyquist criteria (sampling rate more than twice the highest frequency in the signal) then it won't be a factor in the accuracy of the RMS value.
Since you are measuring velocities the nyquist criteria could be a problem if you don't know the frequencies involved. The frequencies would be related to the acceleration - higher acceleration means higher frequencies. I don't know how to tell you to determine the needed sampling rate give the expected acceleration.
If you can't determine the highest frequency (or are limited in your sampling frequency,) then you might run your sampling at the highest available rate, but filter the analog signal with a lowpass with a cutoff frequency lower than half of the sampling rate. This will avoid the problems that arise when sampling a fast signal with a too slow sampling rate, at the expense of throwing away information if the sampling rate really is too slow.
The number of samples only comes into it when trying to calculate the RMS value of a repeating signal, line a sine wave. In that case you have to make sure to get a least one full cycle of the lowest frequency. More could help make up for inaccuracies in the measurement or reduce some of the noise. In this case, the longer sampling period averages out any measurement errors and noise.
Summary: Make sure that your sampling is fast enough, that the samples are evenly spaced, and that you (obviously) have all the samples for the period of interest.
