You can use autofill to create a series of numbers, letters, or dates in Google Sheets.
Use autofill to complete a series
- On your computer, open a spreadsheet in Google Sheets.
 - In a column or row, enter text, numbers, or dates in at least two cells next to each other.
 - Highlight the cells. You’ll see a small blue box in the lower right corner.
 - Drag the blue box any number of cells down or across.
    
- If the cells form a series of dates or numbers, the series will continue across the selected cells.
 - If the cells don’t form a series of dates or numbers, the list of values will repeat across the selected cells.
 
 
Tip: You might notice autofill suggestions with a preview of the values. To accept the suggestion, press Command ⌘  Enter.
Turn off autofill suggestions
1. On your computer, open a spreadsheet in Google Sheets.
2. At the top, click Tools  Suggestion controls.
3. Deselect Enable autocomplete.