nTx denotes the number of antenna ports. The antenna ports correspond to the “antennas” in the digital domain.
One antenna port accumulates the signal from multiple antenna elements, where static, complex-valued weights are applied on the signal from each element, yielding the electrical downtilt.
Example: If you talk about a 4×2 MIMO system, then your transmitter has 4 antenna ports. It could have, e.g., 40 antenna elements, with 10 elements per antenna column mapped to one antenna port.