What is the relation between noise (AWGN) variance and number of antennas?
Usually, the model is that every receiver antenna picks up noise of the same PSD, $N_0/2$. This means that the variance of noise samples in each quadrature branch is $N_0/2$. This noise is local to each antenna and is independent of the rest of the system.

To calculate the total amount of noise in the receiver, you multiply by the number of receiver antennas. This is usually not needed, though, since the figure of merit is the ratio of the energy per information bit (or symbol) and the variance of the noise affecting that bit (or symbol).