Try ":FzfLua serverlist" if you use fzf-lua github.com/ibhagwan/fzf-l… . It lists (and previews) all running Nvim sessions (+ remote sessions, if their local sockets live in stdpath('run')).
It also shows a "last activity" column (Nvim 0.13 "v:useractive").
It also shows a "last activity" column (Nvim 0.13 "v:useractive").