The above mentioned example of having 5 output channels for the demux can be realized as follows.
Since the number of channels = 5, the number of select bits = 3
(As 2^3 = 8 > 5, whereas if we consider select bits = 2, then 2^2 = 4 < 5)
Consider the output channels as C1, C2, ...C5
Here you can see that based on the bits S2, S1, S0 the output channel is selected.
Since we have only 5 channels, the remaining 3 combinations of the select bits is assigned as "don't care" elements. We can either assign the demux to choose any particular channel if any of these 3 combinations occur or we can use these combinations as RESET codes so that when the demux receives one it resets the demux, thereby skipping one data input cycle (no output for that particular cycle)