How to calculate the depth of sorting networks? I have trouble understanding how to calculate the depth of a sorting network on $n$ inputs.

For example, in case of selection sort, we have:

$\qquad \displaystyle D(n)=D(n-1)+2\\\qquad D(2)=1$

which leads to

$\qquad \displaystyle D(n)=2n-3=\Theta(n)$

I have confirmed that the depth of selection sort is equal to $2n-3$ by hand, but I can't understand how the recurrence $D(n)=D(n-1)+2$ is derived.
For selection/bubble sorting network algorithms, the main tricky point is that "some operations are overlapped" or in other words because of pipeling! (it is shown in figure below)
You can find more information in these lecture notes by Rolf Karlsson.

This is the reason why the time complexity is linear!