I tend to disagree on the Parallel Process part... after all, it's being made to run several of them parallel ^^ However, it IS true that depending on their starting condition, events will take more or less processing power, as PPs and atuostart events have their contents executed as well.
Definately, testing your game on several machines is a good thing... I, for example, can run a bunch more of events than 50 (don't really have an exact number), but just having a strong machine doesn't mean you can run infinitely many events, as they're quite poorly coded and will take a lot of resources to process.
Additionally, I'd like to mention that you can somewhat counterfire this by using something called an Anti Event Lag script, or just Anti Lag script... you'll find several among the forums. These restrict the number of updated events to a certain ratio around the visible screen, meaning no matter what size your map has, only that area is taken into consideration when updating events. Because of PP and autostart events, you definately want to grab one that allows you to define no-matter-what updating events, so you can place them anywhere on the map still.