You can only have one solution open at a time. For example you may be working on a complex DLL (composed of multiple sub-projects compiling to static libs) as one solution. You may also want to have the solution for a consuming application open so you can test changes there as well.
I don't use the visual studio IDE for my work (just msbuild from the command line) but if I did it would be more than 5 solutions that I use daily. In rare cases I need to enlist in even more projects. I'm not the original OP but I can certainly emphasize with his problem.
That said my work gets done on a Xeon workstation not a laptop, so in that respect we are different.