Is there a limit of the number of items in a list or documents in a library that SharePoint Collector can export?

There is no limit to the number of items in a list or library that SharePoint Collector can export. SharePoint Collector does not have any limits other than the OS file system limits. See SharePoint documentation of the limits of SharePoint itself – https://support.office.com/en-us/article/SharePoint-Online-limits-8f34ff47-b749-408b-abc0-b605e1f6d498