Creating a Project Number each time you create one forces you to keep track of the numbers in other page/window/document.
Even though it is alpha-numeric, it can be done automatically.
Take a look a Billable app. They ask you to create a letter code for each client and the numbering format of your preference; then the alphanumeric value is generated automatically each time. You can change it if you want, but they do the initial work for you.
Un abrazo,
Robby!