In the loop, use a 2D array for f which depends on i and k:
for k = 1 : 6
for i = 1 : length(X)
f(k, i) = the function that takes k and X(i) as input arguments
end % of loop over i
end % of loop over k
Or, just in case you got your k and i mixed up, you can do it over only k
for k = 1 : 6
f(k) = the function that takes k and X(k) as input arguments
end % of loop over k
Use whichever way you were thinking of.