You may use the Code of A Ninja - Twitter ID Finder (https://codeofaninja.com/tools/find-twitter-id/) to find your Twitter Numeric ID. Type in your username, press Get Twitter ID, and your Twitter Numeric ID will appear on the screen.
Note: We are not affiliated with Code of a Ninja, but their website is helpful for users to acquire their Twitter Numeric ID, so we've mentioned it here. There may be other sites that provide these services; conduct a search on Google and do not login if any website asks you to do so.