Recently, I have been asked by a number of organisations about the maximum amount of Public Folders that can be migrated to Exchange Online. This used to be set in stone at 100k pre-September 2017 and required significant planning to reduce the number of Public Folders that could be migrated.
The Exchange Team Blog announced back in April 2018 of an increase in the amount of public folders you can migrate from Exchange 2010 had increased to 250k, allowing you to expand this to 500k post migration to Exchange Online. See here for more information.
As of now, Microsoft also support the migration of up to 250k public folders from Exchange 2013/2016/2019 also, again allowing you to expand this to 500k post migration to Exchange Online. See here for more information.
This will allow many organisations to migrate more Exchange 2010/2013/2016/2019 Public Folders to Exchange Online and plan less to reduce them in size.