How do I make a single mob spawn?

You should be able to tweak the spawn weight to a good level, but it sounds like your spawn group max is too high. Set both the min and max entities in your spawn group to 1 in the 'spawning' tab.

Create a global logic variable to track when herobrine spawns in.  When herobrine spawns set the variable to true, then use the wait (number) ticks then do on server side block to waite a long time before using the despawn event/target entity and setting the variable to false.  Also add in a custom spawn condition so that herobrine can only spawn if the variable if false, something like:  

If is light at x y z less than 7 and variable = false

        do  Return true

Return false

A side effect of this is that herobrine will randomly disappear, but I hope that that is not too bad.