i counted 18 decoders in 2 stages (using 3line to 8 line decoders)
16 decoders in the second stage (16 x 8 = 128 wires can be tested) with the select inputs of all these 16 decoders shorted together
and 2 decoders in first with the select inputs shorted together
so we have 6 select lines - 3 from stage 1 and 3 from stage 2
each decoder in the first stage controls the enable inputs of 8 decoders in the second stage
the enable inputs of the 2 decoders first stage are controlled with i/o lines from the MCU
six more i/o lines are needed three for first stage select inputs (msb) and 3 for second stage select inputs (lsb)
hope it is clear enough - let me know if you need any other details