function get_field_length(field) -- checks how many entries are in a list
l=1
k=0
while (l~=nil) do
k=k+1
l=field[k]
end -- while
return (k-1)
end -- function

field={1,3,2,5,4}
FL=get_field_length(field)
print("Field length is ",FL)

field = {1, 3, 2, 5, 4}
print('Field length is ', #field)

This works for most tables.

2.5.5 - The Length Operator

The length operator is denoted by the unary operator #. The length of a string is its number of bytes (that is, the usual meaning of string length when each character is one byte).

The length of a table t is defined to be any integer index n such that t[n] is not nil and t[n+1] is nil; moreover, if t[1] is nil, n can be zero. For a regular array, with non-nil values from 1 to a given n, its length is exactly that n, the index of its last value. If the array has "holes" (that is, nil values between other non-nil values), then #t can be any of the indices that directly precedes a nil value (that is, it may consider any such nil value as the end of the array).