Max amount would depend on the number of multiplexers you wanted to connect up.
For example, you could use a part like DG406
If the photodiodes work okay with the DG406 on-resistance of 100 ohm, then you can connect up 16 to this part, and 16 to the other 5 analog inputs, and have 96 inputs.
You will need 4 output pins to select 1 of of 16 outputs from each chip. Write a simple loop:
apply address 0000, read 6 inputs (1 from each mux)
apply address 0001, read 6 inputs
:
:
apply address 1110, read 6 inputs
apply address 1111, read 6 inputs
repeat