how many select lines are required for a 1-to-8 demultiplexer?

A 1-to-8 demultiplexer requires 3 select lines. This is because the number of select lines mmm needed is determined by the formula:

2m=number of outputs2^m=\text{number of outputs}2m=number of outputs

For 8 outputs:

2m=8 ⟹ m=32^m=8\implies m=32m=8⟹m=3

Thus, 3 select lines are needed to select one of the 8 output lines for the single input