A 12 bar blues is 3 4 bar phrases.
The 1st 4 bars is nothing but the 1 chord, sometimes the 4 chord is inserted into measure 2 for variety.
The next 4 bars is 2 measures of the 4 chord, 2 measures of the 1 chord
The next 4 measures are 5 4 1 usually followed by a 4 to 5 turnaround.
In traditional blues each of the 4 bar phrases would contain 1 vocal line. The same vocal line would be repeated in phrases 1 & 2 with an answer/response followed in phrase 3.
ex.
I'm a king bee baby, I want you to be my queen
I'm a king bee baby, I want you to be my queen
We can make honey baby, like the worlds never seen