How many periods per week must a primary school teacher cum subject head teach?

According to Clause 1, Article 6 of the Working Regime for General Education Teachers issued in conjunction with Circular No. 28/2009/TT-BGDDT, the teaching hour norms are specified as follows:

The teaching hour norm is the number of theoretical or practical hours each teacher must teach in a week. To be specific:

The teaching hour norm for primary school teachers is 23 hours, for lower secondary school teachers is 19 hours, and for upper secondary school teachers is 17 hours;

And in Clause 5, Article 8 of the Working Regime for General Education Teachers issued in conjunction with this Circular, policies on reducing teaching hour norms for teachers concurrently performing specialized tasks are specified as follows: Subject leader teachers are reduced by 3 hours/week.

Thus, the teaching hour norm for primary school teachers is 23 hours/week. However, since you are concurrently the head of the Mathematics subject group, you will be reduced by 3 hours/week. Therefore, based on the above regulation, you will teach 20 hours in a week.

Please refer to the following article for more details:

How many hours can homeroom teachers in charge of subject classrooms be reduced?

Best regards!