At what dew point is there a risk of mould?

It is not the dew point alone that matters but its distance to the coldest surface in the room. If the wall temperature is below the dew point, water condenses; mould already grows from about 80 % humidity at the surface. At 20 °C and 60 % room humidity (dew point 12 °C), cold corners at 13 to 14 °C are already at risk. Keep surfaces at least 2 to 3 °C above the dew point.