I am a bit confused about what mailbox(es) need to be created for proxy access to the on prem Exchange 2010 public folders.
https://docs.microsoft.com/en-us/exchange/collaboration/public-folders/configure-legacy-public-folders-for-hybrid?view=exchserver-2019
Step 5 states
"You will point to all of the proxy public folder mailboxes that you created in Step 2: Make remote public folders discoverable to enable the Exchange Online organization to access the on-premises public folders."
In step 2 you create an empty mailbox database and populate it with a proxy mailbox. Is only one proxy mailbox required to facilitate access to all legacy public folders on a single host?