Type `1` in the first cell
Type `2` in the next cell
Select both cells
Drag the fill handle down to continue the series
Use the formula `=ROW()-ROW($A$1)` in the first data row
Copy the formula down to fill serial numbers automatically
Use the formula `=SEQUENCE(COUNTA(A:A)-1)` in supported Excel versions
Convert the range to a table and use a formula-based serial number column
Use AutoFill with a pattern if you want continuous numbering
Use `=COUNTA($A$2:A2)` to generate serial numbers based on filled rows