Hi Laure, the Student license limit of 128K is based on the highest node or element number in the Mechanical model. This includes elements that are created automatically for you such as Contact elements, so it is not just the elements you can see in the mesh. Sometimes, there is a gap in the element numbering so the Statistics on the number of elements in the model is a smaller number than the highest element number. You can insert a Numbering(?) object under the Mesh and force it to renumber the mesh to eliminate gaps.
In Fluent, the Student license limit is 512K nodes or cells. Fluent can compute heat transfer for solids as well as fluids, but it's a whole different interface.