disclaimer- people may have already made some or all of these points on this post. im not wading through the trite to check How many players do you think can work in an event?
realistically this just depends on how many ET there are running the event. take however many players the average ET can reasonably handle in any given situation, and multiply it by the amount of ET. its basic geography
Did you like the old 'bounty board' monster hunting events? yes, it made it so that people who wanted to do monster **** could go 'hey lets organize a time with so-and-so and have them do an event for us!!' and stopped the semi-annoying although not too common 'lol xd wild monster appears' that were far too common in events i saw back in the fringe
What is your opinion on PvE in events? depends on the usage, if you're eventing them through a dungeon and dont want to spend upwards of thirty minutes having a group of ten people fight monster a and monster b then pve is fine, although it should be taken into account who the event is being done for and whether or not they actually like mechanics combat elements at all
What do you think of allowing some events, such as dungeons and fights, to be repeatable to allow different groups to experience the same event? i think it's fine in a general sense with something like a monster board you could simply say 'hey there isn't just one of these monsters!!!' and thats really not stretching anyones suspension of disbelief unless it's some crazily specific monster, or just a person. (like the hag event, why would upwards of three or four magic witch ladies move into the exact same house??? if the locations themselves were different it'd be fine on that front)
What is your opinion on large event storylines? How can they be improved? theyre good, as long as theyre accessible without being forced. present the events in a logical, story driven manner and if the players latch on to this overarching story between certain events, then good job, you did it, if they dont, sucks. some people will like the thought put into the event(s), and most people wont because one off events with no real correlation to anything are easier to digest and enjoy in a moment to moment basis TL;DR
player count should be decided based on context and the kind of event occurring, dont force events on people who clearly dont want events done at them, orchestrate good stories if you want people to be interested, dont beat people over the head with the point, and use PvE when its deemed more helpful than would-be convoluted RP combat **** fests