How many coats of Unitex render do I need to apply prior to applying Unitex texture?

This depends on the substrate. But you need to apply as many coats of render as needed to achieve a true and level and smooth sponged surface acceptable for the final surface applied finish and paint coats.

Note: keying coats are generally 1 to 3 millimetres thick.  Build levelling coats are generally 5 to 10 millimetres. And sponge float finishes are generally 1 to 2 millimetres.