Assume that 4 molecules of nitrogen and 9 molecules of hydrogen are present? after complete reaction, how many molecules of ammonia are proceded?

2 N2 + 3 H2 →2N H3

That's the balanced equation. As shown, 4 molecules of N2 would react with 12 molecules of H2. Since not that much H2 molecules exist, then it would serve as the limiting reactant. Therefore:

9 mol H2 * (2NH3 / 3H2) = 6 molecules of ammonia