Whilst there is no maximum the number of individual assets you can have on hold in the archive, there is a maximum number of assets you can place on hold at any one time from the search results. This limitation is 1,000,000 assets at any one time.
How do I apply a hold? Please click this link HERE:
What do I need to do if the search I have, contains more than 1,000,000 assets (search results) and I want them all on hold?
Duplicate/Copy the existing saved search (how to duplicate/copy an existing saved search, click HERE:)
Apply a date range to the search, to reduce the number of results, so that it does not exceed 1,000,000 results in total, apply the required hold reason (noting that this is a background activity, it could take several hours to complete placing all items on hold from any search submitted, additionally we will only process one submitted request at a time for your account, any other tasks will be placed into a queue for processing when the other completes)
Replicate this action until you have all the results spread across multiple searches and are able to apply the hold upon them.