There is no restriction on the number of distinct ports that may be assigned in one plane. The following image shows three independent wave port assignments.
All three of these ports share the same port plane. The port solve eigenanalysis extracts three modes for this port plane, and combines them to satisfy the voltage requirements for field output. You may see the fields resulting from the uncombined eigenmodes by creating a port field annotation. The following three images show the real part of the electric field on the port corresponding to the three eigenmodes. As you can see, these modes include all the wave ports on the port plane, and they do not satisfy the voltage requirements.
You can see the combined modes by creating a volumetric field annotation. The following three images show the volumetric fields for the three ports. For these results, the eigenmodes have been combined to satisfy the voltage requirements. Specifically, in the first image only port 1 has non-zero voltage; in the second image only port 2 has non-zero voltage, and so on. The three ports are stimulated independently.