1. Is there any recommended limit of images to be published by a multiple selection?
2. Is there any recommended limit of images contained in a single folder?
1. There should be no such limit, however, there is a known limitation in the UI which limits any action performed to a maximum of 100 assets. This issue is reported in the internal Jira request - ASSETS-10808 and the engineering team is actively working on fixing this.
Moreover, replication happens in batches by default, so should be ok in terms of server load. It does however mean that if it’s a large number of files it could hog your replication queue until the process is done.