The more elements people have to process, the easier for them to make a mistake. On average people can rember between 5 and 9 items, with 7 being the magic number most designers use. Instead of focusing on that number so much, however, it's good to simply reduce complexity in a system overall.
Split tasks into easy to follow, logical steps instead of adding in everything at once.
Every additional element on the screen will require some processing time. The more "busy" our brain gets, the bigger the chance for a mistake or abandoning the process altogether.
If you have any research evidence, that this tip does not work in specific conditions, please share it with us. We will expand the database with that particular case to grow the collective knowledge together.