Finding number of independent samples using autocorrelation
The autocorrelation you obtained shows that the signal becomes decorrelated with itself for specific time delays, which are the zero-crossings of the autocorrelation. The autocorrelation is not zero except at these time instants.

You can also define a threshold $\tau$ and say that, if the autocorrelation is less than $\tau$, then for your purposes the signal is decorrelated for any time shift $\Delta$ for which the autocorrelation is less than $\tau$.

When calculating the autocorrelation in Matlab (or similar), the lag is calculated in number of samples. For example, in your case it seems like the autocorrelation is zero for a lag of 200. Since your sampling interval is $T_s = 1/5000 = 200\,\mu$s, then your signal becomes decorrelated for a shift of 200 samples, or a time shift $\Delta = 200 T_s = 40\,$ms.