There is no exact limit on number of assets to upload. But there is a pragmatic limit as many different factors limit the processing power of our environment . Adobe is increasing throughput continually.
Run large-scale uploads (for example 50,000 images) in off-peak (nonbusiness hours). Discuss initial image loads (for example, for a new customer uploading hundreds of thousands of images) with Tech Support or your implementation consultant. They can advise on the most effective setup.
See the Help instructions under "Upload files using Via FTP." The instructions mention how to choose the FTP folder from which to upload files. Make sure to select "Include Subfolders" if you want to upload the contents of the subfolders under the source folder and create the same subfolder under the destination folder.