Artur,
Thank you for the clarification. We don't actually have a minimum of levels required to run WRF. The input data come with a set number of vertical levels and then, depending on what you set the number to when running real/wrf (e_vert), the model will just interpolate to however many levels you requested, using the data from the available levels from input. This is actually not always great practice, as sometimes the number of levels doesn't provide enough information between levels for a good interpolation. Unfortunately, it's just something each person will need to test - to make sure things look reasonable. I'd recommend running a short test with the set-up you're interested in and see whether it looks okay. 15 levels may be okay, but we typically see closer to 30 levels for basic input data.
Kelly