Does anyone know if it is possible to have a build number which is unique to projects in TeamCity.
I know there is build.number which is sequential and unique per build configuration.
There is also teamcity.build.id which seems to be sequential and unique to the whole of TeamCity.
The reason I want a number per project is for packaging and archiving purposes. Want to be able to package and store as ProjectName/BuildNumber but can't do this if multiple build configurations on the same project produce the same build numbers.
It is, indeed. You'll need to use the autoincrementer plugin.
Though the plugin page doesn't say so, it is compatible with TC10 (which I think is the version you're interested in, based on the question tag).