Sequence Number With Criteria Contains (single Or Multiple Data)

i am looking some formula to make sequence number with criteria;

1) if in cell B2 contains only single/one data ----the result is mark "-"

2) if in cell B2 contains several data (not single)----the result is auto numbering with adding mark "." (dot) and start from 1.,2.,3.,4,etc........

for the detail,
condition 1 (multiple data in col.B)

number
name

1.
john

2.
mike

3.
sisca

etc...

condition 2 (single data in col.B)
number
name