No, there is no limit—you can select all objects at once for attachment migration. For example, as shown in the screenshot below, 159 objects have been selected in a single migration session.
PS: View Mass File Migrate Salesforce file migration app on Salesforce AppExchange:
https://appexchange.salesforce.com/appxListingDetail?listingId=e9874a66-2001-4e3d-8a29-3a2ed7f584c4