Short answer: Yes, you can upload multiple files in a zip file as long as all the files are relevant supporting documents to your copyright material.
For the copyright to be effectively verified, you can only upload a single file through the copyright submission form. For example, a screenplay in pdf format or a song in mp3 format can be uploaded directly as single files. In many cases, a body of work needs to be registered under one copyright. A body of work is defined as multiple files that constitute a single copyright or supporting documents directly related to a single copyright.
Good examples of a body of work:
A musician registering an album that would consist of multiple mp3 songs and possibly a cover artwork image before sharing it with labels.
A screenwriter registering multiple episodes of a season of a TV series screenplays before sending it out to studios and networks.
A photographer registering multiple photographs of a single tennis match of Rafa Nadal and Roger Federer before sending them out to news outlets.
Bad examples of a body of work:
A screenwriter registering multiple of his screenplays that are unrelated to each other (i.e. they are not sequels, prequels or trilogies).
A designer registering designs of multiple vessels under one copyright.
A musician registering hundreds of unpublished songs and demos that accumulated throughout the years with different collaborations that are unrelated to each other.
Please note that the maximum upload size is 1GB with the possibility of extending to 5GB.
