i am working on fun fur scarves and one of the scarves i want to do red and black i have a pattern but i want to do half black and half red and i have to cast on 13 ST so the question is how many rows do i do till i add the red

So what is your question?

my question is how many rows should i do till i add the red

Depends what length you want. If you mean you want one half of the entire scarf to be black and one half red, then knit with the black until you’re at half the length you want. This is done with a tape measure, not counting rows.

the scarf i am making is 3 inches x 104 inches but i want to do like 10 rows of back then 10 rows of red

If you keep track of your rows as you knit them (like maybe on a piece of paper), you’ll know when to switch colors–after row 10, 20, 30, etc.

Just count the rows then, switch colors after every 10 rows.