I use several (sometimes overlapping) solutions to contain a collection of related independent applications and shared libraries. As others have mentioned, you really don’t want to have a single Git repository containing the source for multiple, independent projects as it makes it much too difficult to track isolated changes.
So, if your solution is structured as mine is then you will definitely want individual Git repositories for each project. This has worked well for me for ten to twelve applications and doesn’t create as much maintenance overhead as you might think.
If your solution is truly monolithic (and you’re sure you want it that way forever and ever), then it probably makes sense to only have a single repository.