I might get a b, how much will this affect me?

I am currently a freshman taking the following core classes: PreAP English, Bio, Integrated Math 2 (the equivalent of algebra 2), and I have already fulfilled my freshman history/social studies requriment. I realized that I won’t be able to pull off an A in my math class and will probabaly get a B+ as my final grade. To be clear, I am not taking freshman math, I am taking a sophmore class. I know that grades aren’t everything on college applications but I an interested in more selective schools such as Stanford. Other than academics, I participate in 4 school clubs and associations and am on the board of 2. I am also on a sports team. I plan on letting this be my only b in my high school career if possible. How much will this affect me in the long run?