OneDrive vs SharePoint - when should a shared folder in OneDrive be moved to a SharePoint site?

As Pydel mentioned, it's not how many files necessarily. It's who, what and where.
If you think of the files in the traditional paper filing cabinet sense, what does that look like? Does each person on the team have their own filing cabinet, and everyone shares a little with all the others? Do I have to remember that Sally has one file I want, Bill has another, and John has yet another? Each one of them would then have to share with the individuals on the team, and each individual would have to remember where everything is. SharePoint allows for that one-stop location, without loss of business continuity when team members leave or new ones join.
you can overcome some of this by picking one person that stores all of the files in a single location, and then shares with the team, but what happens when that person leaves? For a team, solely using OneDrive has the potential of being an organizational nightmare.
Check out Matt Wade's jumpto365 where he answers what to use when
https://www.jumpto365.com/blog/which-tool-when-sharepoint-onedrive-or-microsoft-teams

Good luck