fundamental limitations
You run into some fundamental and some practical limitations.
The gain of an array of radiators is the product of the individual radiator gain and the array factor. This is the absolute maximum gain. The array factor is set by the number and spacing of the individual radiators. The higher the gain of an individual radiator, the greater the spacing needed. One way to visualize this is the receiving mode and capture area. If the capture areas overlap, you do not get the full effects of the number of receptors.
The practical limit is the cumulative losses and phase errors of the tree of splitters used to feed the radiators.