I usually go with a set of related software and their dependencies in a single repository. Just because I find it easier to manage.
Those repositories fall into two categories:

• A repo with one main program and a set of supporting smaller programs.
  
• A repo with a set of equally small, but related and interdependent, programs;
  

If, in the first category, one small program grows sufficiently larger to steal the spotlight of the main program, I may consider breaking the repo in two.