First time Box braids!

I'm gonna get box braids in a few weeks and idk how much hair I should buy my hair straightened is at boob level I want my braids to be mid back I was looking online at samsbeauty.com where I get my full units. And I came across the hair used for braiding and I want to mix it with 2 different tones of blonde but idk how much I should buy if someone can help me that'd be nice! Also when I actually go get them done idk what size I should say but I'd want it like this