@supeertin_ generally 6 rows is about 1 cm of string so for this pattern itś 14.5cm but you will need more than that for the base strings. I would use a minimum of 20cm for each base string. If you want to make it into a bracelet though, I would make the base strings at least 30 cm each. Now for the white string that will be showing will use a lot, so I would not cut the thread until the end. When you buy the thread, it should be 800cm. I would just use all of the 800cm until the alpha is completed and cut off any excess string at the end.
@supeertin_ do you know how to make alpha patterns? If not I recommend watching MashaKnots tutorial on the flat alpha technique.
For this pattern you need 12 basestrings (colour doesn't matter) and thread in the colours white, brown and two shades of green for the leading strings