I would like to know whether we can build multiple projects through a single build.xml file(Ant) or do we need to have different files for different projects.
No, the build.xml file is fully conformant with the XML standard, which specifies exactly one root node, no more, no less.
However, you can have multiple build xml files and have the master build.xml call them. You can name them whatever you want (build.xml is just the default name).
Education won't help those who are proudly and willfully ignorant. They'll literally rather die before changing.