Project Numbers are assigned by project type and are not unique. This means that you can have duplicate Project Numbers for projects of differing types. The way these projects are differentiated in the database is by the Project ID which is unique to each project regardless of project type.
Unfortunately, there is no way to change the project numbers that have already been assigned.
There is a suggestion we can make, so that they are more unique going forward.
There is a field in each project type in the admin zone named 'Next Item Number'.
You can give each project type a unique starting number and any future projects will begin with that number.