The number 786 means 786 and nothing more!
When Allah or His messenger want to teach us something, they do so in the clearest way, far from puzzles and mystery numbers. All the efforts made by some Muslims to count letters and relate them to numbers are nothing but a waste of time. Sometimes they do so with good intention to show that our deen is great. Indeed our deen is great, and not in need of such erroneous approaches.