I am working on some WC partitions, my boss wants the total thickness to be 50mm (including 6mm tiles and 3mm adhesive on both sides). So the work and the wediboards should be a total of 32mm.

How can I achieve a 50mm wall total?

If you use 6.25 mm boards that gives a gap of 19 mm. You can then use 18-20 mm studs between the boards. May not be exact but close. Not sure what size boards or studs are available. In US imperial measurements that translates to 1/2 inch boards and 3/4 in studs.

You need to ask this in the country you buy materials in. In the US, 'fiber cement' board comes in thicknesses that vary by brand (Hardie, wonder board, Durorock). .42", 7/16", 1/2", and thinner for putting over a substrate.

(I would buy ready made metal partitions with chromed feet. It will pass building and health codes and make for much more sanitary mopping and cleaning of walls. Partitions will need a lot of support, including to the ceiling, especially with tile. You don't want to use wood for any of it, covered or not. Tile is extremely sensitive to the slightest movement, and will crack too easily, at least the grout, if not the tile itself too.)