Most of my projects are "on hold" or put in the Someday folder. New projects default to "on hold" unless it needs to be done in the very near future. Then it becomes an "active" project for me to work on until the due date.
In addition to projects, I have lists for various areas of responsibilities. I have a Home Miscellaneous list, Work Miscellaneous list, and numerous other lists that gather up single actions (not projects).
A weekly and monthly review allows me to drop off Someday projects that are no longer important or needed.
When I finish 1 active project, I can go to my Someday folder and activate an "on hold" project.
Some projects can be deferred to a future date. I have a Christmas shopping project that is scheduled to start November 1st. It stays in my Someday folder for most of the year and I never have to touch it. My "File the 1040 taxes" project starts on January 5th and is completed on or before April 15th.
I try to keep my active projects list down to a minimum. At this moment, I see I have 3 work projects, 1 home project, and 2 projects for my sideline job as active. Everything else is scheduled into the calendar or in the Someday folder.
But, of course, we all have our working threshold of how many active projects we can have running simultaneously. If they can keep on top of all that, go for it! Otherwise, I'm the kinda guy who tries to keep just enough active projects. I have daily work and customer requests that can fill up my "Office Miscellaneous" tasks and my wife's Honey-Do list to keep me busy. This prevents my active projects list from growing any larger.
YMMV