"I used the genetic algorithm, in general idea about this method ,the convergence and the number of iteration is related by what?"
No-one knows. There have been successful theoretical analysis for genetic algorithm convergence for some kinds of functions, and some functions appear to converge in practice, but for other functions it is not known whether genetic algorithms converge at all.
"In other hand how to solve the problem of convergence(number of gradients) ?"
On the scale of that diagram, the search looks like it might have converged. But remember that there is a large difference between "convergence" and "finding the global minima". You can easily construct functions where the global minima cannot be found through any search algorithm:
f = @(x) 1000 - (x == 0.1688799713442523664497230129200033843517303466796875)
