Every arabic letter has a numerical value. The numerical values of the arabic letters of Bismillahir Rahmaanir Rahim added up totals to 786. Hence, using "786" is not a baseless practice and cannot be deemed haraam as it has not been prohibited in the Sharia. However, this practice is not established in any narration nor is it equivalent to writing Bismillah in full. However it is used on certain occasions to avoid disrespect to the name of Allah.
And Allah Ta'ala Knows Best