I'm the only core contributor to the code. There have always been people helping a bit here and there, especially in the early days. But, there has been no one as committed to the project as I have been. There are more than 130 contributors to the main repository. But they were mainly providing small pull requests.
Although I'm the only developer working directly on the fullPage.js code, I have a small team of people helping me with other things – all of them working part-time, most just a few hours a week.