I haven't encountered a limit, but just note that each event added counts as 1 handle, and as such, it occupies memory. You can clear those handles only by destroying the trigger.
That's why there are damage detection systems out there--the damage event requires you to input a particular unit, so to detect *any* damage from any unit, you must register the event for all units on the map. You can make a trigger to add the "takes damage" event whenever a unit enters the map, but it is a bit difficult to handle what happens when a unit is removed or dies. For some maps, you might not even have to worry about it (if your map doesn't use too many units, e.g. a hero arena). For others, it will matter more (e.g. footman frenzy). Damage detection systems will destroy the damage detection trigger and rebuild it occasionally to prevent the events from adding up indefinitely.