How much tile adhesive do i need?

Based on the formula of 1.8 kgs per 1mm depth per m2, you will need to allow a minimum 3mm bed of adhesive for bonding down the xps premium or heatstore tile backer boards to the sub-floor,
4mm coverage of the cable plus a minimum 1.5mm bed of adhesive to bond the tiles (total depth of adhesive = 8.5mm).

Example for a 5m2 tiled area with insulation boards

5m x 8.5mm x 1.8kg = 77kg Total amount of adhesive = 4 x 20kg bags

Example for a 10 m2 tiled area:

10 x 8.5 x 1.8 = 153 kgs. Total amount of adhesive = 8 x 20 kgs.

This Depends on what floor finish you are having, if your having a tiled finish then you dont always need to latex the cables/mats, sometimes its easier to pour a thin bed of latex on them to hold them in position for when you come to tile it.

If you are having a vinyl floor then you will need to totally embed the cables in latex, this will normally require at least 6mm of levelling compound.
based on the formula
1.67Kg/sqm x area(sqm) x 6mm thickness divided by 20Kg = number of bags

Example for a 5m2 area

1.67kg x 5m x 6mm = 50 kg Total amount of adhesive = 3 x 20kg bags

Example for a 10m2 area

1.67 x 5 x 6 = 100kg Total amount of adhesive = 5 x 20kg bags