Version 11.5 runs on macOS 10.13 High Sierra (later minor updates added support for 10.14 Mojave and limited support for Catalina). A Fusion 11 Professional licence would let you run Fusion 11.0.x or 11.5.x on a single computer. If you buy VMware Fusion 12 Professional (not Player), it includes the right to downgrade the licence to Fusion 11 Professional. Fusion 12 requires macOS 10.15 Catalina or later.
VMware only sells the current version of Fusion, which at present is Fusion 12 (Player or Professional). As far as I'm aware, there is no longer any way to get a new or upgrade licence for Fusion 10 - that version was discontinued several years ago.
