Just do as instructions ask. Link: https://www.irs.gov/pub/irs-pdf/i5500ez.pdf

Line 5a(1). Enter the total number of participants at the beginning
of the plan year.
Line 5a(2). Enter the total number of active participants at the
beginning of the plan year.
Line 5b(1). Enter the total number of participants at the end of
the plan year.
Line 5b(2). Enter the total number of active participants at the
end of the plan year.