Generally speaking, the traditional brush calligraphy is written vertically from right to left, and there is no need to arrange it according to the lines of poetry. If it is a banner, you can write two lines, four poems and so on.
If you write with a hard pen, you can write with a traditional vertical banner or a horizontal banner. In horizontal style, for neatness, you usually write in lines of poetry.
However, there are no absolute rules, as long as the written layout is beautiful.