In each of the topologies we usually assume a single ISL between switches. In some applications it may be necessary to have multiple inter-switch links (ISLs) between switches for reasons of bandwidth or resilience.
Remember, a SAN is a solution to a particular requirement, so that in practice the optimal design for a particular application may be a combination of Cascade, Mesh and/or Core-Edge topologies, and may not need to be a fully symmetrical design.