Since the establishment is not a factory but a commercial establishment in Tamil Nadu (I presume), the Tamil Nadu Shops and Commercial Establishments Act will be applicable to your establishment. As such, you have to give 12 days of sick leave and 12 days of casual leave in a year. Additionally, after the completion of 12 months' service, the employee should be given 12 days of earned leave as well. Although sick leave and casual leave would lapse at the end of the year (normally, the calendar year is considered as the year for this purpose), the unavailed earned leave can be carried forward to the next year, accumulating up to 45 days.
An employee joining at any time during the year shall be given casual leave or sick leave proportionately. Therefore, if an employee joins a company in August, they should be credited with 5 days each of casual leave and sick leave for the year up to December. Since the employee would earn Privilege Leave only after 12 months' service, they may be given credit for it only in 2025. However, if you want to establish a uniform date for crediting the leave, you can credit the earned leave also in January, but the credit shall be made proportionately.
Unlike the Factories Act, reference is not made about physical days worked under the Shops Act. Therefore, leave is credited based on months only. As such, the leave (casual leave and sick leave) is to be credited in advance and not after every month. In practice, when you take a month for calculating the leaves, if the date of joining is prior to the 15th of the month, i.e., any day between the 1st and the 15th, it would be counted as one month, and any date of joining after the 15th would be ignored. Therefore, an employee joining on the 25th would not get leave for that month.
The number of privilege leaves that you give is more than what is required under the law. I hope you have casual leave and sick leave also in force over and above the privilege leave mentioned in the post.