For example, if you have three projects called proj1, proj2, and proj3, you can easily build all three at once by typing the following at the DOS/command prompt:
DCC32 proj1.dpr proj2.dpr proj3.dpr
If you're using Delphi 1.0, use DCC instead of DCC32:
DCC proj1.dpr proj2.dpr proj3.dpr