You need to look at Section 317 of Companies Act 2014.
The way this works is that if an employee is in existence for any time in that month then they are considered 1/12th of an employee. You look at it on a monthly basis. So if that person worked 3 hours in every month in the year, that person would be considered 1 employee in the year. You add all of these together for each month and divide by 12.