I have requirement to upload thousands of assets in MM. I have browsed docs.sdl.com but could not found any resource for bulk uploading. is there any UI/API available to upload the multiple images or any other optimized way to achieve it?
There is a Media Manager Web Service available with which you can create logic to bulk upload items. The CHM file in the page has information on how to upload items to Media Manager. Pay special attention to the Sdl.MediaManager.Service Namespace section and then the IMediaManager2011.GetUploadUrl method description. It has examples on how the upload mechanism works and can programmed against.
You need to ensure that you have the WCF bindings set up correctly. The web service uses WS 2007 Federation Binding, which you have to connect to a Secure Token Server to make it work. This means you also have to use a client machine which has access and can use the Media Manager Certificates.