Typically you will encounter this when you create a new Post and you are not giving the post a title until after the Autosave feature has already saved your initial post. If you don't give your post a Title, WordPress will assign a number when it Autosave's your post. Since the permalink has been "officially" set, when you actually enter a Title the permalink remains as the number.
To fix this, always make sure you update the permalink before publishing your post...or enter a title first, before you start typing out the content for that post.
Before you Publish your Post, check the permalink...if it is a number > click Edit > Erase the number > click OK. WordPress will update it based on your title > then update / publish the post.