"... making the life of millions of people happier ..." or "... making the lives of millions of people happier ..."
The first one could be interpreted as "every person lives one single joint life", while the second one could mean that everyone has more than one life.
But of course I would like to say the "third option in between", that is everyone has one life, but the number of lives equals to the number of people, so there is not only a single life everyone shares.
What is the correct structure for saying that? Can I pick one from the aforementioned two so that it won't be ambiguous, or do I need to use some different structure?