There is no specific number stated in either the Act or the Regulation.
The employer must consult employees in relation to the number of employee and employer representatives on any OHS committee. (Regulation - Clause 22(2)(e))

The number of employer representatives on the OHS committee must not exceed the number of elected employee representatives (Clause 24 (d)).

A person is not eligible to be an employer representative on an OHS committee unless the person has authority to act on behalf of the employer on OHS matters. (Clause 24(j)).

Consultative arrangements (including membership aspects of the OHS committee) must take into account the composition of the workgroup.