I prefer to work at one at a time, but it all depends and a lot of that is how much fabric is out/required. Once the cutting is done and I'm working from a kitted project I'm more flexible. I don't really have any UFOs, but at least one of my "in process" tops is getting close to that designation. I'm hoping to spend time with it this year, it involves paper piecing and I've been working with that to build up to going back to that one.
I keep my projects pretty well organized by keeping them in a project box throughout the process. My "in process" project is in a tub that sits on top of my sewing surface, silently reproaching me for neglect. I cover it so it doesn't get sun faded, but I know It's there.... I get it out and mess around a bit and then put it back in the box.
Sometimes I deliberately have multiple projects, designed so I can have a to-go project, like for something to do during my Tuesday group meetings. It's a motivator finding something that can be done for two hours every week! One of the reasons I do my pinning the way I do is it was one of the only portable stages for my work, first started that at the group before this one, so back around Y2K.
PS: I used to complete each quilt including quilting before moving on to the next one and worked that way for decades. I no longer consider "completed tops" as UFOs. They are complete, just not quite quilts yet.