How do I know when I need dentures?

Function. When you lose the function of your teeth because of loose teeth (periodontaly hopeless), and there are no alternatives to a denture that you can afford or do. Most patients after they have a denture built and fitted, can chew and enjoy there food better than eating around many loose, hurting, and non functional teeth. Also the removal of such infected teeth improves your overall health.
No teeth. If you have no teeth, toyu need dentures.
Easy. When the teeth left in your mouth and not adequate for chewing and eating, if they interfere with other functions of your mouth such as speech, if your teeth or roots are infected, or if you have any other sort of gum disease. Please see the appropriate dental professional for evaluation.
Total tooth loss. Getting complete dentures is meant for total tooth loss. When it comes to dentures, you should go to a dentist or denturist.