Number of antenna to perform MUSIC algorithm
In the field of array signal processing, the signal model is
$$
\mathbf{x}(n) = [x_0(n), x_1(n), \ldots, x_{M-1}(n)]^T
$$
where $M$ denotes the number of sensor and $n$ is the discrete time.
While in the field of spectral estimation, the signal model is
$$
\mathbf{x}(n) = [x(n), x(n-1), \ldots, x(n-M+1)]^T
$$
where $M$ is the number of samples of the observed signal, and $x(n-m)$ represents the previous signal before $m$ samples.
The autocorrelation matrix is
$$
\mathbf{R}_{xx} = E\{ \mathbf{x}(n) \mathbf{x}^H(n) \}
$$
and it is usually replaced by its estimation $\mathbf{\hat{R}}_{xx}$ in practice.