These will usually be a line or two of lyrics, or a set melody structure you can write to. That is how I have proceeded as a performer working with other performers; it is central to the work. In this regard, I am grateful to Pattison for getting me to think. It varies; in theory you can add as many verses as you want.
This is the only element which is a hundred percent essential to a song. Don't go too far off topic, and start getting ready to end the song.
Read more about adding emotion to your lyrics here. Your verses should have a good reason to hang out together. Then rewrite the third line B line to target a different vowel sound in the title. Prosody is about rhythm, stress, even pitch inflection in speech.