Also you can see that first and last base string is never used in the pattern (as all the knots are black) so you can use any color for these strings, although it looks better when there are only two colors in the braids.
When doing a two color alpha you need to decide which color will be your background (the one that is used more). In this case the background color is black 🙂 for the background you need one long black string. your base is white so you will need 9 white strings 🙂