I have two. Early career I found the second one absolutely improved my productivity - perhaps by 50% or more - as it helped me multitask really effectively.
Now, later in my career I have had kids for a while. My multitasking went out the window when I had kids - I find it hard to juggle more than one or maybe two things I’m working on at a time. I suspect this was due to poor sleep - parents never seem to really catch up to sleeping full nights like before kids. Instead of multitasking on lots of small things I transitioned to more in-depth work where I can focus for longer periods on a single thing.
Now, I think having a second monitor is still useful but I can function fine without it. It’s maybe a 10% boost if that.
Phone: great for mindlessly scrolling or the odd comment.
Laptop: for actually getting anything done.
I’d use a desktop but sometimes I have to work from cafes or something so I prefer just using a laptop all the time rather than two machines