Made this per someone's request. Added the trigger functionality because I noticed to right click a spawner, one must be OP oddly enough. Otherwise, the PlayerInteractEvent doesn't fire. Also the spawn event will tell you if a mob spawned due to a spawner, but sadly doesn't provide a conduit back to said spawner. So the only way I can correlate mobs to their spawners is to check if they are within 16 blocks of said spawner. I read that 16 is the distance on a wiki somewhere. However, in case I'm wrong, I made that adjustable in the configuration.

I designed this under 1.17 as that's what my test server is, but should work on any modern Spigot version.

Commands:

/SUR SET number
sur.set
Enters selection mode to set a spawner as a user limited spawner. Number is the minimum amount of players that have to be on to allow the selected spawner to work.

/SUR DEL
sur.del
Removes the user requirement from a spawner.

/SUR LIST
sur.list
Lists all spawners that have user requirements.

/SUR TRIGGER trigger
sur.trigger
Sets whether left or right click is the trigger to select a spawner during set or del.

/SUR RANGE number
sur.range
Sets the distance from the spawner with which to correlate a mob spawn to said spawner.

/SUR RELOAD
sur.reload
Reloads configuration