The manual says:

   The Emacs server can optionally be stopped automatically when
   certain conditions are met.  To do this, call the function
   @code{server-stop-automatically} in your init file (@pxref{Init
   File}), with one of the following arguments...

It'd be nice if this were a defcustom so that people who prefer the 
Customize interface could use that instead of editing their init files. 
Here's a patch for that.

I've tried to make sure this is as robust as possible, so that 
everything is always properly set, especially when adjusting the value 
in the Customize UI. It should even work correctly if a user stopped the 
server temporarily and restarted it (you might do this to clear out old 
clients, do some work, and then make the server available again). Maybe 
this last bit is a little paranoid, but it was minimal extra work 
compared to getting the Customize part working.

One question though: should this only go on the master branch, or should 
it go into the 29 branch? To me, it seems like it could go either way, 
though I think it'd be nice to make this easier for users in 29. I'll do 
whatever the maintainers think is best though.

If it goes on the master branch only, I'll add back the function form of 
'server-stop-automatically' for compatibility, and then mark it obsolete.