the years as a system admin taught me that you can identify the influential contributors because they were the only people whose accounts were not immediately shut off when the management identified a bus factor.
and now i have a name for the phenomena; thank you.
You're talking about work slowing because of increased overhead from more people needing to communicate and make decisions.
The OP is talking about the"bus factor". How many people can leave the project unexpectedly and still have the project survive. E.g. if only one person has access to merge changes, the bus factor is 1 regardless of how many people actively contribute.
If the lessons that I've learned about lightbulb replacement are applicable, then the nationality of the developers on the bus will impact the answer to your question.