Is there a limit to how many activity types we should create?
Although I am not sure what the exact impact on performance would be, it does not sound as a good idea. An activity can already be linked to a campaign, to make specific types for each campaign or event sounds like a bit of a nightmare.

Have you considered using the CiviRules extension (https://civicrm.org/extensions/civirules and documentation on https://docs.civicrm.org/civirules/en/latest/) for sending these emails? I reckon that might make life a little easier than creating hundreds of activity types, specific for each campaing or event.