Comment by imtringued

Probably 0. That number will grow significantly you directly ask contributors during hacktoberfest if they want to become long term contributors.

There are four types of contributors. Project maintainers and team members who are invested in the project. Contributors who want a bug or feature implemented and will do it themselves. Those who want to contribute to opensource projects but can't decide on which project or issue to work on but hacktoberfest gives them an excuse to just pick any project and if you ask them directly they might stick. Finally there are those who only do it for external rewards.

The people you can reach only through hacktoberfest are the last two groups and only the third group might stay over the long run. The first two groups will always be there, even without hacktoberfest.