There’s no limit in the plugin itself.
But you’re limited by number of data you can save at once and how long this operation can perform.
And these are PHP related parameters like:
max_input_vars – defaults to 1000, number of input variables may be accepted
max_execution_time – the maximum script execution time
Changing these values should do the trick.
But honestly speaking the number of files you’re trying to attach is huge and there are different ways to do it. For that number of attachments we’d rather create custom taxonomy for media, provide a way to relate a post to attachment via taxonomy terms and populate the attachments list dynamically via Download Attachment available functions. This would require some playing around with custom code, but would be much more stable and performant imo.
