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The kernel device /dev/mice has worked since at least 2001 and maybe earlier. This device takes the input from all mice and presents it as a Microsoft Explorer PS/2 protocol.
If you or your distro set X to use /dev/mice then hotplugging any number of mice worked flawlessly.
The only exceptions were special devices like the Space Orb, multi-ball trackballs, mice with more than 7 buttons, etc.