Answer
(Fatwa: 153/110/L=1434)
786 is the number of Bismillah (بسم اللہ الرحمن الرحیم) and it indicates towards it. It is sunnah, wajib, fardh or bida’h to write it. Yes, it is allowable to write only 786 if someone fears that by writhing Bismillah the name of Allah may be desecrated, but obviously the virtues of Bismillah shall not be gained from this number.
Note: At the time of writing 786 one should say Bismillah so that it is started with the same.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband