What does the number after Vitamin B signify?
It appears to be fairly arbitrary, but they were originally numbered chronologically by discovery. However...

From Gizmodo:


  These Bs are not necessarily designated in chronological order, as B12 (Cobalamins) was discovered in 1926, B5 (Pantothenic acid) and B7 (Biotin) in 1931, B6 (Pyridoxine) in 1934, B3 (Niacin) in 1936 and B9 (Folic acid) in 1941. The missing Bs are substances originally thought to be vitamins, but later reclassified.


Wikipedia explains that certain numbered B vitamins were originally classified as other vitamin letters when discovered and only later reassigned as a B vitamin. 

In conclusion, B vitamins were numbered in order as they were assigned as a B vitamin.


missing numbers are due to specific vitamins being reclassified as either different letters or as non-vitamins.