200+ phototransistors and reading them
One approach would be to multiplex your ADC with lots of analog multiplexors (muxes).

Another approach is to connect all phototransistor collectors together to one ADC channel.  Connect the emitters to digital i/o pins.  If the i/o pin is floating, the phototransistor is not driving the ADC.  If the i/o pin is low, the phototransistor is driving the ADC.  This allows you to select one phototransistor at a time.  You would need as many digital i/o lines as there are phototransistors.  In this approach, you trade analog muxes for digital i/o.

Somewhere in between is a matrix approach.  Suppose, you have 256 phototransistors.  You could arrange them in a matrix.  8x32, for example.  8x ADC channels (or 1x ADC channel with 8-to-1 analog mux).  One for each row of the matrix.  All collectors in one row are connected together.  32x digital i/o.  One for each column.  All emitters in one column are connected together.  Digital i/o works in the same way as in the 2nd paragraph, except one i/o activates a column of 8x phototransistors.