Hi Camilo
Note: please don't use capital C as a variable, it is reserved in Wolfram language
Please try this
B[C_, alfa_, x_] := (x*Sqrt[(105*C)/(8*Pi*(10 - 3*alfa))])/
Sqrt[gmod[C, alfa, x]];
cList = {0.33282, 0.338, 0.34322, 0.34848, 0.35378, 0.35912, 0.3645, 0.36992};
FindMaximum[B[#, 0, x], {x, 1}] & /@ cList
Also look again at the documentation
FindMaximum