I list as many things as come to mind so as not to forget anything important. I then select the three most important things to do today.
It usually ranges between 3 to 5. The number depends on what was pending from yesterday plus what are the tasks for today plus any thing I need to prepare in advance for tomorrow. I also prioritise my tasks in the list so that my focus remains with important work when the to-do list is long.
It depends on the day. Some days are more than others. I try to at least put a minimum of 3 things, including things I want to do and have to do.