Here is a list of all the Sequence Number formulas you can use. Refer to this article for more help on how to insert the formula into a field.
Simple Sequence Number
SequenceNo
Output: 1
Sequence Number not starting at 1
SequenceNo+65
Output: 66
Sequence Number with a Prefix
CONCATENATE("PR",SequenceNo)Output: PR1
Prefixed Sequence Number with Padded Zeros
CONCATENATE("PR",TEXT(SequenceNo,"00000"))Output: PR00001
Sequence Number with Year
CONCATENATE(YEAR(CreatedAt),"-",SequenceNo)
Output: 2017-1
Sequence Number with 2 Digit Year
CONCATENATE(TEXT(CreatedAt,"yy"),"-",SequenceNo)
Output: 17-1
Sequence Number with Month as an Abbreviation
CONCATENATE(TEXT(CreatedAt,"mmm"),"-",SequenceNo)
Output: Mar-1
Sequence Number with Month as a Single Digit
CONCATENATE(MONTH(CreatedAt),"-",SequenceNo)
Output: 3-1