12 Periods: March 2020 - Feb 2021 - If you select 'Make Last Period as First Period of Next Year'. Feb2021 will be the period 1 of the new financial year.
13Periods: March 2020 - March 2021- If you don't select 'Make Last Period as First Period of Next Year'. April 2021 will be the period 1 of the new financial year.
Based on the above, if you only want to run 12 periods. Don't select the option 'Make Last Period as First Period of Next Year'.
If you wish to run 13 Periods. Select the option 'Make Last Period as First Period of next Year'.
If you don't set up this option correctly, you'll find that your periods are incorrect after running the year end.